diff --git a/main.zig b/src/main.zig
similarity index 100%
rename from main.zig
rename to src/main.zig
diff --git a/src/wordle.zig b/src/wordle.zig
new file mode 100644
index 0000000..f9c08f9
--- /dev/null
+++ b/src/wordle.zig
@@ -0,0 +1,97 @@
+/// The length of Wordle words.
+const WordLen = 5;
+
+/// A single Wordle word (ASCII).
+const Word = [WordLen]u8;
+
+/// The type of string literals representing words.
+const WordLiteral = *const [WordLen:0]u8;
+
+/// A clue for a spot given a guess.
+const Clue = enum {
+    Gray,
+    Yellow,
+    Green,
+};
+
+/// The list of clues for a full guess.
+const Clues = [WordLen]Clue;
+
+/// Guesses a word given a solution and returns the clues.
+pub fn checkGuess(guess: Word, solution: Word) Clues {
+    // init clues
+    var clues: Clues = undefined;
+
+    // compare each letter pairwise
+    for (guess, 0..WordLen) |guessLetter, idx| {
+        // if letters match, the clue is green
+        if (guessLetter == solution[idx]) {
+            clues[idx] = Clue.Green;
+            continue;
+        }
+
+        // default to a gray clue
+        clues[idx] = Clue.Gray;
+
+        // if a matching letter in the solution can be found, write yellow
+        for (solution, 0..WordLen) |solutionLetter, _| {
+            if (guessLetter == solutionLetter) {
+                clues[idx] = Clue.Yellow;
+                break;
+            }
+        }
+    }
+
+    // return completed clues
+    return clues;
+}
+
+/// Converts a fixed-length string literal into a word.
+fn strToWord(str: WordLiteral) Word {
+    var buffer: Word = undefined;
+    @memcpy(&buffer, str);
+    return buffer;
+}
+
+/// Asserts that two word literals produce the expected clues.
+fn expectGuessClues(guess: WordLiteral, solution: WordLiteral, expectClues: WordLiteral) error{Mismatch}!void {
+    // convert literals to the non-null word type
+    const guessWord = strToWord(guess);
+    const solutionWord = strToWord(solution);
+    const expectCluesWord = strToWord(expectClues);
+
+    // check the guess and produce clues
+    const clues = checkGuess(guessWord, solutionWord);
+
+    // confirm that the expected clues match the clues
+    for (clues, 0..) |clue, idx| {
+        // convert character to associated clue variant
+        const expected = switch (expectCluesWord[idx]) {
+            ' ' => Clue.Gray,
+            'Y' => Clue.Yellow,
+            'G' => Clue.Green,
+            else => unreachable,
+        };
+
+        // if clues don't match, return an error
+        if (expected != clue) {
+            return error.Mismatch;
+        }
+    }
+}
+
+test "full match" {
+    try expectGuessClues("crane", "crane", "GGGGG");
+}
+
+test "one off" {
+    try expectGuessClues("crane", "crank", "GGGG ");
+}
+
+test "no match" {
+    try expectGuessClues("salet", "fuzzy", "     ");
+}
+
+test "duplicate letters" {
+    try expectGuessClues("fuzzy", "zesty", "  Y  ");
+}