1 files changed, 125 insertions, 0 deletions
diff --git a/discover.go b/discover.go
new file mode 100644
index 0000000..ee9c2e6
--- /dev/null
+++ b/discover.go
@@ -0,0 +1,125 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io/fs"
+	"os"
+	"path/filepath"
+	"regexp"
+	"sort"
+	"strings"
+)
+
+type Skill struct {
+	Name string
+	// Path is the skill's directory relative to the source root.
+	Path        string
+	Description string
+}
+
+var validName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)
+
+// discover finds every skill in root: a directory holding a SKILL.md. It does
+// not look for skills nested inside another skill.
+func discover(root string) ([]Skill, error) {
+	var skills []Skill
+	seen := map[string]string{}
+	err := filepath.WalkDir(root, func(path string, d fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+		if !d.IsDir() {
+			return nil
+		}
+		if path != root && (strings.HasPrefix(d.Name(), ".") || d.Name() == "node_modules") {
+			return filepath.SkipDir
+		}
+		data, err := os.ReadFile(filepath.Join(path, "SKILL.md"))
+		if err != nil {
+			return nil
+		}
+		rel, _ := filepath.Rel(root, path)
+		fm := frontmatter(data)
+		name := fm["name"]
+		if name == "" {
+			name = filepath.Base(path)
+		}
+		if !validName.MatchString(name) {
+			return fmt.Errorf("%s/SKILL.md: invalid skill name %q", rel, name)
+		}
+		if prev, ok := seen[name]; ok {
+			return fmt.Errorf("skill %q is defined twice, in %s and %s", name, prev, rel)
+		}
+		seen[name] = rel
+		skills = append(skills, Skill{Name: name, Path: filepath.ToSlash(rel), Description: fm["description"]})
+		return filepath.SkipDir
+	})
+	sort.Slice(skills, func(i, j int) bool { return skills[i].Name < skills[j].Name })
+	return skills, err
+}
+
+// frontmatter reads the top-level scalar keys of a SKILL.md's YAML front
+// matter. That is all skills need from it, so it skips a YAML dependency.
+func frontmatter(data []byte) map[string]string {
+	fm := map[string]string{}
+	sc := bufio.NewScanner(bytes.NewReader(data))
+	if !sc.Scan() || strings.TrimSpace(sc.Text()) != "---" {
+		return fm
+	}
+	for sc.Scan() {
+		line := sc.Text()
+		if strings.TrimSpace(line) == "---" {
+			break
+		}
+		if line == "" || line[0] == ' ' || line[0] == '\t' {
+			continue
+		}
+		k, v, ok := strings.Cut(line, ":")
+		if !ok {
+			continue
+		}
+		v = strings.TrimSpace(v)
+		if len(v) >= 2 && (v[0] == '"' || v[0] == '\'') && v[len(v)-1] == v[0] {
+			v = v[1 : len(v)-1]
+		}
+		fm[strings.TrimSpace(k)] = v
+	}
+	return fm
+}
+
+func selectSkills(found []Skill, only []string) ([]Skill, error) {
+	if len(only) == 0 {
+		return found, nil
+	}
+	byName := map[string]Skill{}
+	for _, s := range found {
+		byName[s.Name] = s
+	}
+	var out []Skill
+	for _, n := range only {
+		s, ok := byName[n]
+		if !ok {
+			return nil, fmt.Errorf("no skill named %q (found: %s)", n, skillNames(found))
+		}
+		out = append(out, s)
+	}
+	return out, nil
+}
+
+func skillNames(skills []Skill) string {
+	names := make([]string, len(skills))
+	for i, s := range skills {
+		names[i] = s.Name
+	}
+	return strings.Join(names, ", ")
+}
+
+func skillPaths(skills []Skill) map[string]string {
+	m := make(map[string]string, len(skills))
+	for _, s := range skills {
+		m[s.Name] = s.Path
+	}
+	return m
+}