package skillpack

import (
	"bufio"
	"bytes"
	"fmt"
	"strings"

	"gopkg.in/yaml.v3"
)

// ManifestName is the required filename at a pack's root.
const ManifestName = "SKILL.md"

// Limits on manifest fields, matching the Anthropic agent-skills constraints so
// packs authored against that ecosystem validate here unchanged.
const (
	maxNameLen        = 64
	maxDescriptionLen = 1024
	maxBodyBytes      = 1 << 20 // 1 MiB of instruction text is already excessive
)

// Manifest is a parsed SKILL.md: YAML frontmatter plus the markdown body. Only
// Name and Description are required; everything else is optional and passes
// through so a host can honor it (or ignore it) without this package growing a
// policy opinion.
type Manifest struct {
	// Name is the pack's stable identifier (kebab-case, unique within a host's
	// subscriptions). It is what the model passes to skill_use.
	Name string
	// Description is the one-liner shown in the catalog — the ONLY text loaded
	// into the prompt up front, so it must convey when to reach for the skill.
	Description string
	// License is an optional SPDX-ish tag, informational only.
	License string
	// AllowedTools is the pack author's declared tool allow-list. It is advisory
	// here: a host MAY intersect it with the agent's real toolset, but it can
	// only ever NARROW, never grant (see the host wiring, not this package).
	AllowedTools []string
	// Metadata is arbitrary passthrough frontmatter (e.g. version) the host may
	// use; this package does not interpret it.
	Metadata map[string]string
	// Body is the markdown instruction text after the frontmatter — the payload
	// skill_use returns on demand.
	Body string
}

// ParseManifest parses a SKILL.md byte slice into a validated Manifest. The
// input must begin with a `---` YAML frontmatter block; the remainder is the
// body. It returns a descriptive error on malformed frontmatter or a field that
// violates the limits, so a bad pack fails loudly at subscribe/sync time rather
// than silently activating.
func ParseManifest(raw []byte) (*Manifest, error) {
	front, body, err := splitFrontmatter(raw)
	if err != nil {
		return nil, err
	}

	// Decode into a permissive intermediate: SKILL.md uses hyphenated keys
	// (allowed-tools) and lets metadata values be scalars of any type.
	var fm struct {
		Name         string         `yaml:"name"`
		Description  string         `yaml:"description"`
		License      string         `yaml:"license"`
		AllowedTools stringList     `yaml:"allowed-tools"`
		Metadata     map[string]any `yaml:"metadata"`
	}
	if err := yaml.Unmarshal(front, &fm); err != nil {
		return nil, fmt.Errorf("skillpack: invalid SKILL.md frontmatter: %w", err)
	}

	m := &Manifest{
		Name:         strings.TrimSpace(fm.Name),
		Description:  strings.TrimSpace(fm.Description),
		License:      strings.TrimSpace(fm.License),
		AllowedTools: []string(fm.AllowedTools),
		Body:         strings.TrimSpace(string(body)),
	}
	if len(fm.Metadata) > 0 {
		m.Metadata = make(map[string]string, len(fm.Metadata))
		for k, v := range fm.Metadata {
			m.Metadata[k] = fmt.Sprintf("%v", v)
		}
	}

	if err := m.Validate(); err != nil {
		return nil, err
	}
	return m, nil
}

// Validate reports the first field that violates the manifest contract.
func (m *Manifest) Validate() error {
	switch {
	case m.Name == "":
		return fmt.Errorf("skillpack: SKILL.md missing required 'name'")
	case len(m.Name) > maxNameLen:
		return fmt.Errorf("skillpack: name %q exceeds %d chars", m.Name, maxNameLen)
	case !isKebab(m.Name):
		return fmt.Errorf("skillpack: name %q must be lowercase kebab-case (a-z, 0-9, -)", m.Name)
	case m.Description == "":
		return fmt.Errorf("skillpack: SKILL.md missing required 'description'")
	case len(m.Description) > maxDescriptionLen:
		return fmt.Errorf("skillpack: description exceeds %d chars", maxDescriptionLen)
	case len(m.Body) > maxBodyBytes:
		return fmt.Errorf("skillpack: body exceeds %d bytes", maxBodyBytes)
	}
	return nil
}

// splitFrontmatter separates a leading `---`-delimited YAML block from the body.
// Leading blank lines/BOM are tolerated. A missing or unterminated block is an
// error — a SKILL.md without frontmatter has no name/description to catalog.
func splitFrontmatter(raw []byte) (front, body []byte, err error) {
	// Strip a leading UTF-8 BOM: editors on some platforms prepend one, and
	// bytes.TrimSpace (used below) does not remove it, so a BOM would otherwise
	// make the first "---" fence unrecognizable.
	raw = bytes.TrimPrefix(raw, []byte{0xEF, 0xBB, 0xBF})
	s := bufio.NewScanner(bytes.NewReader(raw))
	s.Buffer(make([]byte, 0, 64*1024), maxBodyBytes+64*1024)

	var frontLines [][]byte
	var bodyLines [][]byte
	state := 0 // 0=before open fence, 1=in frontmatter, 2=in body
	sawOpen := false
	for s.Scan() {
		line := s.Bytes()
		trimmed := bytes.TrimRight(line, "\r")
		switch state {
		case 0:
			if len(bytes.TrimSpace(trimmed)) == 0 {
				continue // skip leading blanks
			}
			if string(bytes.TrimSpace(trimmed)) != "---" {
				return nil, nil, fmt.Errorf("skillpack: SKILL.md must start with a '---' frontmatter block")
			}
			sawOpen = true
			state = 1
		case 1:
			if string(bytes.TrimSpace(trimmed)) == "---" {
				state = 2
				continue
			}
			frontLines = append(frontLines, append([]byte(nil), trimmed...))
		case 2:
			bodyLines = append(bodyLines, append([]byte(nil), trimmed...))
		}
	}
	if err := s.Err(); err != nil {
		return nil, nil, fmt.Errorf("skillpack: reading SKILL.md: %w", err)
	}
	if !sawOpen || state != 2 {
		return nil, nil, fmt.Errorf("skillpack: SKILL.md frontmatter block is not terminated by a closing '---'")
	}
	return bytes.Join(frontLines, []byte("\n")), bytes.Join(bodyLines, []byte("\n")), nil
}

// stringList decodes either a YAML sequence or a comma-separated scalar into a
// []string, so `allowed-tools: [Read, Bash]` and `allowed-tools: "Read, Bash"`
// both work.
type stringList []string

func (l *stringList) UnmarshalYAML(node *yaml.Node) error {
	var seq []string
	if err := node.Decode(&seq); err == nil {
		*l = trimAll(seq)
		return nil
	}
	var scalar string
	if err := node.Decode(&scalar); err != nil {
		return err
	}
	*l = trimAll(strings.Split(scalar, ","))
	return nil
}

func trimAll(in []string) []string {
	out := in[:0]
	for _, s := range in {
		if t := strings.TrimSpace(s); t != "" {
			out = append(out, t)
		}
	}
	return out
}

// isKebab reports whether s is strict lowercase kebab-case: [a-z0-9] segments
// joined by single hyphens, with no leading, trailing, or consecutive hyphens.
func isKebab(s string) bool {
	if s == "" || s[0] == '-' || s[len(s)-1] == '-' {
		return false
	}
	prevHyphen := false
	for _, r := range s {
		switch {
		case r >= 'a' && r <= 'z', r >= '0' && r <= '9':
			prevHyphen = false
		case r == '-':
			if prevHyphen {
				return false
			}
			prevHyphen = true
		default:
			return false
		}
	}
	return true
}