package step

import "strings"

const frontMatterRule = "---"

// FrontMatter represents optional metadata that can occur at the very start of
// a document.
type FrontMatter = map[string] string

// SplitFrontMatter parses the front matter (if it exists), returning a map
// representing it along with the rest of the input as a string. If there is no
// front matter, an empty map will be returned.
func SplitFrontMatter (input string) (FrontMatter, string, error) {
	if !strings.HasPrefix(input, frontMatterRule + "\n") {
		// no front matter at all
		return FrontMatter { }, input, nil
	}
	input = strings.TrimPrefix(input, frontMatterRule)
	index := strings.Index(input, "\n" + frontMatterRule + "\n")
	if index < 0 {
		return nil, "", ErrFrontMatterNeverClosed
	}
	frontMatterRaw := input[:index]
	body           := input[index + len(frontMatterRule) + 2:]
	frontMatter    := make(FrontMatter)
	for _, line := range strings.Split(frontMatterRaw, "\n") {
		line = strings.TrimSpace(line)
		if line == "" { continue }
		key, value, ok := strings.Cut(line, ":")
		if !ok {
			return nil, "", ErrFrontMatterMalformed
		}
		key   = strings.ToLower(strings.TrimSpace(key))
		value = strings.TrimSpace(value)
		if key == "" {
			return nil, "", ErrFrontMatterMalformed
		}
		if _, exists := frontMatter[key]; exists {
			return nil, "", ErrFrontMatterDuplicateKey
		}
		frontMatter[key] = value
	}
	
	return frontMatter, body, nil
}