Merge pull request 'branched-generated-encoder' (#9) from branched-generated-encoder into message-size-increase

Reviewed-on: #9

design/branched-generated-encoder.md

@@ -0,0 +1,128 @@
+# Branched Generated Decoder
+
+Pasted here because Tebitea is down
+
+## The problem
+
+TAPE is designed so that the decoder can gloss over data it does not understand.
+Technically the protocol allows for this, but I completely forgot to implement
+this in the generated decoder, oops. This would be trivial if TAPE messages were
+still flat tables, but they aren't, because those aren't useful enough. So,
+let's analyze the problem.
+
+## When it happens
+
+There are two reasons something might not match up with the expected data:
+
+The first and most obvious is unrecognized keys. If the key is not in the set of
+recognized keys for a KTV, it should leave the corresponding struct field blank.
+Once #6 has been implemented, throw an error if the data was not optional.
+
+The second is wrong types. If we are expecting KTV and get SBA, we should leave
+the data as empty. The aforementioned concern about #6 also applies here. We
+don't need to worry about special cases at the structure root, because it would
+be technically possible to make the structure root an option, so it really is
+just a normal value. Until #6, we will leave that blank too.
+
+## Preliminary ideas
+
+The first is going to be pretty simple. All we need to do is have a skimmer
+function that skims over TAPE data very, and then call that on the KTV value
+each time we run into a mystery key. It should only return an error if the
+structure of the data is malformed in such a way that it cannot continue to the
+next one. This should be stored in the tape package alongside the dynamic
+decoding functions, because they will essentially function the same way and
+could probably share lots of code.
+
+The second is a bit more complicated because of the existence of KTV and OTA
+because they are aggregate types. Go types work a bit differently, as if you
+have an array of an array of an array of ints, that information is represented
+in one place, whereas TAPE doesn't really do that. All of that information is
+sort of buried within the data structure, so we don't know what we will be
+decoding before we actually do it. Whenever we encounter a type we don't expect,
+we would need to abort decoding of the entire data structure, and then skim over
+whatever detritus is left, which would literally be in a half-decoded state. The
+fact that the code is generated flat and thus cannot use return or defer
+statements contributes to the complexity of this problem. We need to go up, but
+we can't. There is no up, only forward.
+
+Of course, the dynamic decoder does not have this problem in the first place
+because it doesn't expect anything, and constructs the destination to fit
+whatever it sees in the TAPE structure as it is decoding it. KTVs are completely
+dynamic because they are implemented as maps, so the only time it needs to
+completely comprehend a type is with OTAs. There is a function called typeOf
+that gets the type of the current tag and returns it as a reflect.Type, which
+necessitates recursion and peeking at OTAs and their elements.
+
+We could try to do the same thing in the generated decoder, comparing the
+determined type against the expected type to try to figure out whether we should
+decode an array or a table, etc. This is immediately problematic as it requires
+memory to be allocated, both for the peek buffer and the resulting tree of type
+information. If we end up with some crazy way to keep track of the types, that's
+only one half of the allocation problem and we would still be spending extra
+cycles going over all of that twice.
+
+## Performance constraints
+
+The generated decoder is supposed to blaze through data, and it can't do that if
+it does all the singing and dancing that the dynamic decoder does. It's time for
+some performance constraints:
+
+- No allocations, except as required to build the destination for the data
+- No redundant work
+- So, no freaking peeking
+- It should take well under 500 lines of generated code to decode one message of
+reasonable size (i.e. be careful not to bloat the binary)
+
+I'm not really going to do my usual thing here of making a slow version and
+speeding it up over time based on evidence and experimentation because these
+constraints inform the design so much it would be impossible to continue without
+them. I am 99% confident that these constraints will allow for an acceptable
+baseline of performance (for generated code) and we can still profile and
+micro-optimize later. This is good enough for me.
+Heavy solution
+
+There is a solution that might work very well which involves completely redoing
+the generated decoding code. We could create a function for every source type to
+destination type mapping that exists in protocol, and then compose them all
+together. The decoding methods for each message or type would be wrappers around
+the correct function for their root TAPE -> Go type mapping. The main benefit of
+this is it would make this problem a lot more manageable because the interface
+points between the data would be represented by function boundaries. This would
+allow the use of return and defer statements, and would allow more code sharing,
+producing a smaller binary. Go would probably inline these where needed.
+
+Would this work? Probably. More investigation is required to make sure. I want
+to stop re-writing things I don't need to. On the other hand, it is just the
+decoder.
+
+## Light solution
+
+TODO: find a solution that satisfies the performance constraints, keeps the same
+identical interface, and works off the same code. I am convinced this is doable,
+and it might even allow us to extract more data from an unexpected structure.
+However, continuing this way might introduce unmanageable complexity. It is
+already a little unmanageable and I am just one pony (kind of).
+
+## Implementation
+
+Heavy solution is going to work here, applied to only the points of
+`Generator.generateDecodeValue` where it decodes an aggregate data structure.
+That way, only minimal amounts of code need to be redone.
+
+Whenever a branch needs to happen, a call shall be generated, a deferred
+implementation request shall be added to a special FIFO queue within the
+generator. After generating data structures and their root decoding functions,
+the generator shall pick away at this queue until no requests remain. The
+generator shall accept new items during this process, so that recursion is
+possible. This is all to ensure it is only ever writing one function at a time
+
+The functions shall take a pointer to a type that accepts any type like (~) the
+destination's base type. We should also probably just call
+`Generator.generateDecodeValue` directly on user defined types this way, keeping
+their public `Decode` methods just for convenience.
+
+The tape package shall contain a skimming function that takes a decoder and a
+tag, and recursively consumes the decoder given the context of the tag. This
+shall be utilized by the decoder functions to skip over values if their tags
+or keys do not match up with what is expected.

generate/generate.go

@@ -6,6 +6,7 @@ import "maps"
 import "math"
 import "slices"
 import "strings"
+import "encoding/hex"
 import "git.tebibyte.media/sashakoshka/hopp/tape"
 
 const imports =
@@ -33,6 +34,19 @@ type Message interface {
 	// Method returns the method code of the message.
 	Method() uint16
 }
+
+// canAssign determines if data from the given source tag can be assigned to
+// a Go type represented by destination. It is designed to receive destination
+// values from [generate.Generator.generateCanAssign]. The eventual Go type and
+// the destination tag must come from the same (or hash-equivalent) PDL type.
+func canAssign(destination, source tape.Tag) bool {
+	if destination.Is(source) { return true }
+	if (destination == tape.SBA || destination == tape.LBA) &&
+		(source == tape.SBA || source == tape.LBA) {
+		return true
+	}
+	return false
+}
 `
 
 // Generator converts protocols into Go code.
@@ -46,6 +60,14 @@ type Generator struct {
 	nestingLevel int
 	temporaryVar int
 	protocol     *Protocol
+
+	decodeBranchRequestQueue []decodeBranchRequest
+}
+
+type decodeBranchRequest struct {
+	hash [16]byte
+	typ  Type
+	name string
 }
 
 func (this *Generator) Generate(protocol *Protocol) (n int, err error) {
@@ -79,6 +101,14 @@ func (this *Generator) Generate(protocol *Protocol) (n int, err error) {
 		n += nn; if err != nil { return n, err }
 	}
 
+	// request queue
+	for {
+		hash, typ, name, ok := this.pullDecodeBranchRequest()
+		if !ok { break }
+		nn, err := this.generateDecodeBranch(hash, typ, name)
+		n += nn; if err != nil { return n, err }
+	}
+
 	return n, nil
 }
 
@@ -146,7 +176,25 @@ func (this *Generator) generateTypedef(name string, typ Type) (n int, err error)
 	this.push()
 		nn, err = this.iprintf("var nn int\n")
 		n += nn; if err != nil { return n, err }
-		nn, err = this.generateDecodeValue(typ, "this", "tag", "return n, nil")
+
+		nn, err = this.iprintf("if !(")
+		n += nn; if err != nil { return n, err }
+		nn, err = this.generateCanAssign(typ, "tag")
+		n += nn; if err != nil { return n, err }
+		nn, err = this.printf(") {\n")
+		n += nn; if err != nil { return n, err }
+		this.push()
+			nn, err = this.iprintf("nn, err = tape.Skim(decoder, tag)\n")
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateErrorCheck()
+			n += nn; if err != nil { return n, err }
+			nn, err = this.iprintf("return n, nil\n")
+			n += nn; if err != nil { return n, err }
+		this.pop()
+		nn, err = this.iprintf("}\n")
+		n += nn; if err != nil { return n, err }
+		
+		nn, err = this.generateDecodeValue(typ, name, "this", "tag")
 		n += nn; if err != nil { return n, err }
 		nn, err = this.iprintf("return n, nil\n")
 		n += nn; if err != nil { return n, err }
@@ -217,20 +265,25 @@ func (this *Generator) generateMessage(method uint16, message Message) (n int, e
 		n += nn; if err != nil { return n, err }
 		nn, err = this.generateErrorCheck()
 		n += nn; if err != nil { return n, err }
-		abort := "return n, nil" // TODO: skip value somehow
+
-		nn, err = this.iprintf("if !tag.Is(")
+		nn, err = this.iprintf("if !(")
 		n += nn; if err != nil { return n, err }
-		nn, err = this.generateTN(message.Type)
+		nn, err = this.generateCanAssign(message.Type, "tag")
 		n += nn; if err != nil { return n, err }
 		nn, err = this.printf(") {\n")
 		n += nn; if err != nil { return n, err }
 		this.push()
-			nn, err = this.iprintf("%s\n", abort)
+			nn, err = this.iprintf("nn, err = tape.Skim(decoder, tag)\n")
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateErrorCheck()
+			n += nn; if err != nil { return n, err }
+			nn, err = this.iprintf("return n, nil\n")
 			n += nn; if err != nil { return n, err }
 		this.pop()
 		nn, err = this.iprintf("}\n")
 		n += nn; if err != nil { return n, err }
-		nn, err = this.generateDecodeValue(message.Type, "this", "tag", abort)
+		
+		nn, err = this.generateDecodeValue(message.Type, this.resolveMessageName(message.Name), "this", "tag")
 		n += nn; if err != nil { return n, err }
 		nn, err = this.iprintf("return n, nil\n")
 		n += nn; if err != nil { return n, err }
@@ -419,7 +472,10 @@ func (this *Generator) generateEncodeValue(typ Type, valueSource, tagSource stri
 //   - n int
 //   - err error
 //   - nn int
-func (this *Generator) generateDecodeValue(typ Type, valueSource, tagSource, abort string) (n int, err error) {
+//
+// The typeName paramterer is handled in the way described in the documentation
+// for [Generator.generateDecodeBranch].
+func (this *Generator) generateDecodeValue(typ Type, typeName, valueSource, tagSource string) (n int, err error) {
 	switch typ := typ.(type) {
 	case TypeInt:
 		// SI: (none)
@@ -485,49 +541,7 @@ func (this *Generator) generateDecodeValue(typ Type, valueSource, tagSource, abo
 		}
 	case TypeArray:
 		// OTA: <length: UN> <elementTag: tape.Tag> <values>*
-		lengthVar := this.newTemporaryVar("length")
+		nn, err := this.generateDecodeBranchCall(typ, typeName, valueSource, tagSource)
-		nn, err := this.iprintf("var %s uint64\n", lengthVar)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf(
-			"%s, nn, err = decoder.ReadUintN(int(%s.CN()))\n",
-			lengthVar, tagSource)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.generateErrorCheck()
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("*%s = make(", valueSource)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.generateType(typ)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.printf(", int(%s))\n", lengthVar)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("var itemTag tape.Tag\n")
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("itemTag, nn, err = decoder.ReadTag()\n")
-		n += nn; if err != nil { return n, err }
-		nn, err = this.generateErrorCheck()
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("if !itemTag.Is(")
-		n += nn; if err != nil { return n, err }
-		nn, err = this.generateTN(typ.Element)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf(") {\n")
-		n += nn; if err != nil { return n, err }
-		this.push()
-			nn, err = this.iprintf("%s\n", abort)
-			n += nn; if err != nil { return n, err }
-		this.pop()
-		nn, err = this.iprintf("}\n")
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("for index := range %s {\n", lengthVar)
-		n += nn; if err != nil { return n, err }
-		this.push()
-			nn, err = this.generateDecodeValue(
-				typ.Element,
-				fmt.Sprintf("(&(*%s)[index])", valueSource),
-				"itemTag", abort)
-			n += nn; if err != nil { return n, err }
-		this.pop()
-		nn, err = this.iprintf("}\n")
 		n += nn; if err != nil { return n, err }
 	case TypeTable:
 		// KTV: <length: UN> (<key: U16> <tag: Tag> <value>)*
@@ -539,72 +553,7 @@ func (this *Generator) generateDecodeValue(typ Type, valueSource, tagSource, abo
 		n += nn; if err != nil { return n, err }
 	case TypeTableDefined:
 		// KTV: <length: UN> (<key: U16> <tag: Tag> <value>)*
-		lengthVar := this.newTemporaryVar("length")
+		nn, err := this.generateDecodeBranchCall(typ, typeName, valueSource, tagSource)
-		nn, err := this.iprintf("var %s uint64\n", lengthVar)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf(
-			"%s, nn, err = decoder.ReadUintN(int(%s.CN()))\n",
-			lengthVar, tagSource)
-		n += nn; if err != nil { return n, err }
-		nn, err = this.generateErrorCheck()
-		n += nn; if err != nil { return n, err }
-		nn, err = this.iprintf("for _ = range %s {\n", lengthVar)
-		n += nn; if err != nil { return n, err }
-		this.push()
-			nn, err = this.iprintf("var key uint16\n")
-			n += nn; if err != nil { return n, err }
-			nn, err = this.iprintf("key, nn, err = decoder.ReadUint16()\n")
-			n += nn; if err != nil { return n, err }
-			nn, err = this.generateErrorCheck()
-			n += nn; if err != nil { return n, err }
-			nn, err = this.iprintf("var itemTag tape.Tag\n")
-			n += nn; if err != nil { return n, err }
-			nn, err = this.iprintf("itemTag, nn, err = decoder.ReadTag()\n")
-			n += nn; if err != nil { return n, err }
-			nn, err = this.generateErrorCheck()
-			n += nn; if err != nil { return n, err }
-			nn, err = this.iprintf("switch key {\n")
-			n += nn; if err != nil { return n, err }
-			keys := slices.Collect(maps.Keys(typ.Fields))
-			slices.Sort(keys)
-			for _, key := range keys {
-				field := typ.Fields[key]
-				nn, err = this.iprintf("case 0x%04X:\n", key)
-				n += nn; if err != nil { return n, err }
-				this.push()
-					labelVar := this.newTemporaryVar("label")
-					fieldAbort := fmt.Sprintf("goto %s", labelVar) // TODO: skip value somehow
-					nn, err = this.iprintf("if !itemTag.Is(")
-					n += nn; if err != nil { return n, err }
-					nn, err = this.generateTN(field.Type)
-					n += nn; if err != nil { return n, err }
-					nn, err = this.printf(") {\n")
-					n += nn; if err != nil { return n, err }
-					this.push()
-						nn, err = this.iprintf("%s\n", fieldAbort)
-						n += nn; if err != nil { return n, err }
-					this.pop()
-					nn, err = this.iprintf("}\n")
-					n += nn; if err != nil { return n, err }
-					nn, err = this.iprintf("{\n")
-					n += nn; if err != nil { return n, err }
-					this.push()
-						nn, err = this.generateDecodeValue(
-							field.Type,
-							fmt.Sprintf("(&%s.%s)", valueSource, field.Name),
-							"itemTag", fieldAbort)
-						n += nn; if err != nil { return n, err }
-					this.pop()
-					nn, err = this.iprintf("}\n")
-					n += nn; if err != nil { return n, err }
-					nn, err = this.iprintf("%s:;\n", labelVar)
-					n += nn; if err != nil { return n, err }
-				this.pop()
-			}
-			nn, err = this.iprintf("}\n")
-			n += nn; if err != nil { return n, err }
-		this.pop()
-		nn, err = this.iprintf("}\n")
 		n += nn; if err != nil { return n, err }
 	case TypeNamed:
 		// WHATEVER: [WHATEVER]
@@ -619,6 +568,263 @@ func (this *Generator) generateDecodeValue(typ Type, valueSource, tagSource, abo
 	return n, nil
 }
 
+// generateDecodeBranchCall generates code to call an aggregate decoder function,
+// for a specified type. The definition of the function is deferred so no
+// duplicates are created. The function overwrites memory pointed to by the
+// variable (or parenthetical statement) specified by valueSource, and the value
+// will be encoded according to the tag stored in the variable (or parenthetical
+// statement) specified by tagSource. the code generated is a BLOCK and expects
+// these variables to be defined:
+//
+//   - decoder *tape.Decoder
+//   - n int
+//   - err error
+//   - nn int
+//
+// The typeName paramterer is handled in the way described in the documentation
+// for [Generator.generateDecodeBranch].
+func (this *Generator) generateDecodeBranchCall(typ Type, typeName, valueSource, tagSource string) (n int, err error) {
+	hash := HashType(typ)
+	nn, err := this.iprintf(
+		"nn, err = %s(%s, decoder, %s)\n",
+		this.decodeBranchName(hash, typeName), valueSource, tagSource)
+	n += nn; if err != nil { return n, err }
+	nn, err = this.generateErrorCheck()
+	n += nn; if err != nil { return n, err }
+	this.pushDecodeBranchRequest(hash, typ, typeName)
+	return n, nil
+}
+
+// generateDecodeBranch generates an aggregate decoder function definition for a
+// specified type. It assumes that hash == HashType(typ). If typeName is not
+// empty, it will be used as the type in the argument list instead of the result
+// of [Generator.generateType].
+func (this *Generator) generateDecodeBranch(hash [16]byte, typ Type, typeName string) (n int, err error) {
+	nn, err := this.iprintf("\nfunc %s(this *", this.decodeBranchName(hash, typeName))
+	n += nn; if err != nil { return n, err }
+	if typeName == "" {
+		nn, err = this.generateType(typ)
+		n += nn; if err != nil { return n, err }
+	} else {
+		nn, err = this.print(typeName)
+		n += nn; if err != nil { return n, err }	
+	}
+	nn, err = this.printf(", decoder *tape.Decoder, tag tape.Tag) (n int, err error) {\n")
+	n += nn; if err != nil { return n, err }
+	this.push()
+	
+		nn, err = this.iprintf("var nn int\n")
+		n += nn; if err != nil { return n, err }
+		
+		switch typ := typ.(type) {
+		case TypeArray:
+			// OTA: <length: UN> <elementTag: tape.Tag> <values>*
+			// read header
+			lengthVar := this.newTemporaryVar("length")
+			nn, err := this.iprintf("var %s uint64\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.iprintf("%s, nn, err = decoder.ReadUintN(int(tag.CN()))\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateErrorCheck()
+			n += nn; if err != nil { return n, err }
+			elementTagVar := this.newTemporaryVar("elementTag")
+			nn, err = this.iprintf("var %s tape.Tag\n", elementTagVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.iprintf("%s, nn, err = decoder.ReadTag()\n", elementTagVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateErrorCheck()
+			n += nn; if err != nil { return n, err }
+
+			// abort macro
+			abort := func() (n int, err error) {
+				// skim entire array
+				nn, err = this.iprintf("for _ = range %s {\n", lengthVar)
+				n += nn; if err != nil { return n, err }
+				this.push()
+					nn, err = this.iprintf("nn, err = tape.Skim(decoder, %s)\n", elementTagVar)
+					n += nn; if err != nil { return n, err }
+					nn, err = this.generateErrorCheck()
+					n += nn; if err != nil { return n, err }
+				this.pop()
+				nn, err = this.iprintf("}\n")
+				n += nn; if err != nil { return n, err }
+				nn, err = this.iprintf("return n, nil\n")
+				n += nn; if err != nil { return n, err }
+				return n, nil
+			}
+	
+			// validate header
+			// TODO: here, validate that length is less than the
+			// max, whatever that is configured to be. the reason we
+			// want to read it here is that we would have to skip
+			// the tag anyway so why not.
+			nn, err = this.iprintf("if !(")
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateCanAssign(typ.Element, elementTagVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.printf(") {\n")
+			n += nn; if err != nil { return n, err }
+			this.push()
+				nn, err = abort()
+				n += nn; if err != nil { return n, err }
+			this.pop()
+			nn, err = this.iprintf("}\n")
+			n += nn; if err != nil { return n, err }
+
+			// decode payloads
+			nn, err = this.iprintf("*this = make(")
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateType(typ)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.printf(", %s)\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.iprintf("for index := range int(%s) {\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			this.push()
+				nn, err = this.generateDecodeValue(typ.Element, "", "(&(*this)[index])", elementTagVar)
+				n += nn; if err != nil { return n, err }
+			this.pop()
+			nn, err = this.iprintf("}\n")
+			n += nn; if err != nil { return n, err }
+		case TypeTableDefined:
+			// KTV: <length: UN> (<key: U16> <tag: Tag> <value>)*
+			// read header
+			lengthVar := this.newTemporaryVar("length")
+			nn, err := this.iprintf("var %s uint64\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.iprintf("%s, nn, err = decoder.ReadUintN(int(tag.CN()))\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			nn, err = this.generateErrorCheck()
+			n += nn; if err != nil { return n, err }
+			
+			// validate header
+			// TODO: here, validate that length is less than the
+			// max, whatever that is configured to be. if not, stop
+			// ALL decoding. skimming huge big ass data could cause
+			// problems
+
+			// read fields
+			nn, err = this.iprintf("for _ = range int(%s) {\n", lengthVar)
+			n += nn; if err != nil { return n, err }
+			this.push()
+				// read field header
+				fieldKeyVar := this.newTemporaryVar("fieldKey")
+				nn, err = this.iprintf("var %s uint16\n", fieldKeyVar)
+				n += nn; if err != nil { return n, err }
+				nn, err = this.iprintf("%s, nn, err = decoder.ReadUint16()\n", fieldKeyVar)
+				n += nn; if err != nil { return n, err }
+				nn, err = this.generateErrorCheck()
+				n += nn; if err != nil { return n, err }
+				fieldTagVar := this.newTemporaryVar("fieldTag")
+				nn, err = this.iprintf("var %s tape.Tag\n", fieldTagVar)
+				n += nn; if err != nil { return n, err }
+				nn, err = this.iprintf("%s, nn, err = decoder.ReadTag()\n", fieldTagVar)
+				n += nn; if err != nil { return n, err }
+				nn, err = this.generateErrorCheck()
+				n += nn; if err != nil { return n, err }
+
+				// abort field macro
+				abortField := func() (n int, err error) {
+					nn, err = this.iprintf("tape.Skim(decoder, %s)\n", fieldTagVar)
+					n += nn; if err != nil { return n, err }
+					nn, err = this.iprintf("continue\n")
+					n += nn; if err != nil { return n, err }
+					return n, nil
+				}
+
+				// switch on tag
+				nn, err = this.iprintf("switch %s {\n", fieldKeyVar)
+				n += nn; if err != nil { return n, err }
+				for _, key := range slices.Sorted(maps.Keys(typ.Fields)) {
+					field := typ.Fields[key]
+					nn, err = this.iprintf("case 0x%04X:\n", key)
+					n += nn; if err != nil { return n, err }
+					this.push()
+
+					// validate field header
+					nn, err = this.iprintf("if !(")
+					n += nn; if err != nil { return n, err }
+					nn, err = this.generateCanAssign(field.Type, fieldTagVar)
+					n += nn; if err != nil { return n, err }
+					nn, err = this.printf(") {\n")
+					n += nn; if err != nil { return n, err }
+					this.push()
+						nn, err = abortField()
+						n += nn; if err != nil { return n, err }
+					this.pop()
+					nn, err = this.iprintf("}\n")
+					n += nn; if err != nil { return n, err }
+
+					// decode payload
+					nn, err = this.generateDecodeValue(
+						field.Type, "",
+						fmt.Sprintf("(&(this.%s))", field.Name), fieldTagVar)
+					n += nn; if err != nil { return n, err }
+					this.pop()
+				}					
+				nn, err = this.iprintf("default:\n")
+				n += nn; if err != nil { return n, err }
+				this.push()
+					abortField()
+				this.pop()
+				nn, err = this.iprintf("}\n")
+				n += nn; if err != nil { return n, err }
+			this.pop()
+			nn, err = this.iprintf("}\n")
+			n += nn; if err != nil { return n, err }
+			
+			// TODO once options are implemented, have a set of
+			// bools for each non-optional field, and check here
+			// that they are all true. a counter will not work
+			// because if someone specifies a non-optional field
+			// twice, they can neglect to specify another
+			// non-optional field and we won't even know because the
+			// count will still be even. we shouldn't use a map
+			// either because its an allocation and its way more
+			// memory than just, like 5 bools (on the stack no less)
+		default: return n, fmt.Errorf("unexpected type: %T", typ)
+		}
+
+		nn, err = this.iprintf("return n, nil\n")
+		
+	this.pop()
+	nn, err = this.iprintf("}\n")
+	n += nn; if err != nil { return n, err }
+	return n, nil
+}
+
+func (this *Generator) decodeBranchName(hash [16]byte, name string) string {
+	if name == "" {
+		return fmt.Sprintf("decodeBranch_%s", hex.EncodeToString(hash[:]))
+	} else {
+		return fmt.Sprintf("decodeBranch_%s_%s", hex.EncodeToString(hash[:]), name)
+	}
+}
+
+// pushDecodeBranchRequest pushes a new branch decode function request to the
+// back of the queue, if it is not already in the queue.
+func (this *Generator) pushDecodeBranchRequest(hash [16]byte, typ Type, name string) {
+	for _, item := range this.decodeBranchRequestQueue {
+		if item.hash == hash && item.name == name { return }
+	}
+	this.decodeBranchRequestQueue = append(this.decodeBranchRequestQueue, decodeBranchRequest {
+		hash: hash,
+		typ:  typ,
+		name: name,
+	})
+}
+
+// pullDecodeBranchRequest pulls a branch decode function request from the front
+// of the queue.
+func (this *Generator) pullDecodeBranchRequest() (hash [16]byte, typ Type, name string, ok bool) {
+	if len(this.decodeBranchRequestQueue) < 1 {
+		return [16]byte { }, nil, "", false
+	}
+	request := this.decodeBranchRequestQueue[0]
+	this.decodeBranchRequestQueue = this.decodeBranchRequestQueue[1:]
+	return request.hash, request.typ, request.name, true
+}
+
 func (this *Generator) generateErrorCheck() (n int, err error) {
 	return this.iprintf("n += nn; if err != nil { return n, err }\n")
 }
@@ -781,6 +987,19 @@ func (this *Generator) generateTypeTableDefined(typ TypeTableDefined) (n int, er
 	return n, nil
 }
 
+// generateCanAssign generates an expression which checks if the tag specified
+// by tagSource can be assigned to a Go destination generated from typ. The
+// generated code is INLINE.
+func (this *Generator) generateCanAssign(typ Type, tagSource string) (n int, err error) {
+	nn, err := this.printf("canAssign(")
+	n += nn; if err != nil { return n, err }
+	nn, err = this.generateTN(typ)
+	n += nn; if err != nil { return n, err }
+	nn, err = this.printf(", %s)", tagSource)
+	n += nn; if err != nil { return n, err }
+	return n, nil
+}
+
 func (this *Generator) validateIntBitSize(size int) error {
 	switch size {
 	case 5, 8, 16, 32, 64: return nil

generate/protocol.go

@@ -1,5 +1,10 @@
 package generate
 
+import "fmt"
+import "maps"
+import "slices"
+import "crypto/md5"
+
 type Protocol struct {
 	Messages map[uint16] Message
 	Types    map[string] Type
@@ -11,7 +16,7 @@ type Message struct {
 }
 
 type Type interface {
-	
+	fmt.Stringer
 }
 
 type TypeInt struct {
@@ -19,29 +24,84 @@ type TypeInt struct {
 	Signed bool
 }
 
+func (typ TypeInt) String() string {
+	output := ""
+	if typ.Signed {
+		output += "I"
+	} else {
+		output += "U"
+	}
+	output += fmt.Sprint(typ.Bits)
+	return output
+}
+
 type TypeFloat struct {
 	Bits int
 }
 
+func (typ TypeFloat) String() string {
+	return fmt.Sprintf("F%d", typ.Bits)
+}
+
 type TypeString struct { }
 
+func (TypeString) String() string {
+	return "String"
+}
+
 type TypeBuffer struct { }
 
+func (TypeBuffer) String() string {
+	return "Buffer"
+}
+
 type TypeArray struct {
 	Element Type
 }
 
+func (typ TypeArray) String() string {
+	return fmt.Sprintf("[]%v", typ.Element)
+}
+
 type TypeTable struct { }
 
+func (TypeTable) String() string {
+	return "Table"
+}
+
 type TypeTableDefined struct {
 	Fields map[uint16] Field
 }
 
+func (typ TypeTableDefined) String() string {
+	output := "{"
+	for _, key := range slices.Sorted(maps.Keys(typ.Fields)) {
+		output += fmt.Sprintf("%04X %v", key, typ.Fields[key])
+	}
+	output += "}"
+	return output
+}
+
 type Field struct {
 	Name string
 	Type Type
 }
 
+func (field Field) String() string {
+	return fmt.Sprintf("%s %v", field.Name, field.Type)
+}
+
 type TypeNamed struct {
 	Name string
 }
+
+func (typ TypeNamed) String() string {
+	return typ.Name
+}
+
+func HashType(typ Type) [16]byte {
+	// TODO: if we ever want to make the compiler more efficient, this would
+	// be a good place to start, complex string concatenation in a hot path
+	// (sorta)
+	return md5.Sum([]byte(typ.String()))
+}