package doc

import (
	"fmt"
	"strings"
)

// Front is the front matter of a file. There are few keys and all of them are
// flat, so the parsing is done here: pulling in YAML for four `key: value`
// lines earns nothing.
//
// The axis of a layer is declared here by the lang and stack keys rather than
// derived from the path (META-38); the absence of both means the base layer of
// the topic.
type Front struct {
	Topic   string
	Prefix  string
	Lang    string
	Stack   string
	Extends string
	// Origin is the key a copy carries in a consuming repository: the name of
	// the topic it was assembled from. It is the whole front matter of a copy
	// and it appears nowhere in a suite — a file that lost it is no longer a
	// copy and is never overwritten again.
	Origin string

	// At is the line a key was declared on, so a finding can point at the
	// declaration rather than at the top of the file.
	At map[string]int
	// Unknown lists keys the model does not know.
	Unknown []string
	// End is the line of the closing delimiter. The body of the file starts
	// on the next one.
	End int
	// Present says whether there was any front matter at all.
	Present bool
}

// Axis reports whether the layer declares an axis. A layer without one is the
// base layer.
func (f Front) Axis() bool {
	return f.Lang != "" || f.Stack != ""
}

// parseFront parses the front matter out of the file's lines. It returns the
// front matter and the number of the first line of the body.
func parseFront(lines []string) (Front, int, error) {
	front := Front{At: make(map[string]int)}
	if len(lines) == 0 || strings.TrimSpace(lines[0]) != "---" {
		return front, 1, nil
	}
	front.Present = true

	for i := 1; i < len(lines); i++ {
		line := lines[i]
		num := i + 1
		if strings.TrimSpace(line) == "---" {
			front.End = num
			return front, num + 1, nil
		}
		if strings.TrimSpace(line) == "" {
			continue
		}
		key, value, ok := strings.Cut(line, ":")
		if !ok {
			return front, num, fmt.Errorf("front matter line %d is not of the form \"key: value\"", num)
		}
		key = strings.TrimSpace(key)
		value = strings.TrimSpace(value)
		front.At[key] = num

		switch key {
		case "topic":
			front.Topic = value
		case "prefix":
			front.Prefix = value
		case "lang":
			front.Lang = value
		case "stack":
			front.Stack = value
		case "extends":
			front.Extends = value
		case "origin":
			front.Origin = value
		default:
			front.Unknown = append(front.Unknown, key)
		}
	}
	return front, len(lines) + 1, fmt.Errorf("front matter is not closed by a --- delimiter")
}