// Package font parses .font text files and renders bitmap fonts to
// atlases (PNG + JSON metrics) and text images.
package font

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"sort"
	"strconv"
	"strings"
)

// MaxGlyphSize bounds glyph width/height in pixels.
const MaxGlyphSize = 64

// Glyph is one character's bitmap: rows of booleans (true = pixel on).
// All glyphs in a font share the same height; widths vary
// (proportional fonts).
type Glyph struct {
	Char rune
	W, H int
	Rows [][]bool
}

// Font is a parsed .font file.
type Font struct {
	Name       string
	Height     int // glyph height, uniform across the font
	LineHeight int // suggested distance between text baselines
	Baseline   int // rows from glyph top to the baseline
	Spacing    int // horizontal px between glyphs
	SpaceWidth int // advance of ' '
	Glyphs     map[rune]*Glyph
	Order      []rune // file order, for stable atlas layout
}

// ParseFile reads a .font file; the font name defaults to the file name.
func ParseFile(path string) (*Font, error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	ft, err := Parse(f)
	if err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}
	if ft.Name == "" {
		ft.Name = strings.TrimSuffix(filepath.Base(path), filepath.Ext(path))
	}
	return ft, nil
}

// Parse reads the .font text format:
//
//	# comment
//	font: tiny5            optional name
//	spacing: 1             px between glyphs (default 1)
//	space-width: 3         advance of ' ' (default: width of '0' or 3)
//	line-height: 7         default: glyph height + 1
//	baseline: 5            default: glyph height
//
//	glyph A:
//	  .#.
//	  #.#
//	  ###
//	  #.#
//	  #.#
//
// Glyph grids use '#' for on and '.' for off. Every glyph must have the
// same height; widths may differ. The char between "glyph " and the
// trailing ':' is taken literally (one character, e.g. "glyph ::").
func Parse(r io.Reader) (*Font, error) {
	ft := &Font{
		Spacing:    1,
		SpaceWidth: -1, // resolved in validate
		LineHeight: -1,
		Baseline:   -1,
		Glyphs:     map[rune]*Glyph{},
	}
	var cur *Glyph
	sc := bufio.NewScanner(r)
	sc.Buffer(make([]byte, 0, 64*1024), 1024*1024)
	lineNo := 0
	flush := func() error {
		if cur == nil {
			return nil
		}
		if len(cur.Rows) == 0 {
			return fmt.Errorf("glyph %q has no grid rows", string(cur.Char))
		}
		cur.H = len(cur.Rows)
		cur.W = len(cur.Rows[0])
		for i, row := range cur.Rows {
			if len(row) != cur.W {
				return fmt.Errorf("glyph %q row %d is %d px wide, expected %d", string(cur.Char), i+1, len(row), cur.W)
			}
		}
		if cur.W > MaxGlyphSize || cur.H > MaxGlyphSize {
			return fmt.Errorf("glyph %q is %dx%d; the maximum is %dx%d", string(cur.Char), cur.W, cur.H, MaxGlyphSize, MaxGlyphSize)
		}
		if _, dup := ft.Glyphs[cur.Char]; dup {
			return fmt.Errorf("glyph %q defined twice", string(cur.Char))
		}
		ft.Glyphs[cur.Char] = cur
		ft.Order = append(ft.Order, cur.Char)
		cur = nil
		return nil
	}
	for sc.Scan() {
		lineNo++
		line := strings.TrimSpace(sc.Text())
		if line == "" {
			continue
		}
		// '#' starts a comment — except inside a glyph where a line of
		// only '#'/'.' is a grid row.
		if strings.HasPrefix(line, "#") && !(cur != nil && isGridLine(line)) {
			continue
		}
		if strings.HasPrefix(strings.ToLower(line), "glyph ") && strings.HasSuffix(line, ":") {
			if err := flush(); err != nil {
				return nil, fmt.Errorf("line %d: %w", lineNo, err)
			}
			name := strings.TrimSuffix(line[len("glyph "):], ":")
			runes := []rune(name)
			if len(runes) != 1 {
				return nil, fmt.Errorf("line %d: glyph name %q must be exactly one character", lineNo, name)
			}
			cur = &Glyph{Char: runes[0]}
			continue
		}
		if cur != nil && isGridLine(line) {
			row := make([]bool, 0, len(line))
			for _, r := range line {
				row = append(row, r == '#')
			}
			cur.Rows = append(cur.Rows, row)
			continue
		}
		// header key: value
		if i := strings.Index(line, ":"); i > 0 && cur == nil {
			key := strings.ToLower(strings.TrimSpace(line[:i]))
			val := strings.TrimSpace(line[i+1:])
			var err error
			switch key {
			case "font":
				ft.Name = val
			case "spacing":
				ft.Spacing, err = strconv.Atoi(val)
			case "space-width":
				ft.SpaceWidth, err = strconv.Atoi(val)
			case "line-height":
				ft.LineHeight, err = strconv.Atoi(val)
			case "baseline":
				ft.Baseline, err = strconv.Atoi(val)
			default:
				return nil, fmt.Errorf("line %d: unknown setting %q", lineNo, key)
			}
			if err != nil {
				return nil, fmt.Errorf("line %d: %s: %v", lineNo, key, err)
			}
			continue
		}
		return nil, fmt.Errorf("line %d: unexpected %q (want 'key: value', 'glyph X:' or a #/. grid row)", lineNo, line)
	}
	if err := sc.Err(); err != nil {
		return nil, err
	}
	if err := flush(); err != nil {
		return nil, err
	}
	return ft, ft.validate()
}

// isGridLine reports whether the line consists solely of '#' and '.'.
func isGridLine(s string) bool {
	if s == "" {
		return false
	}
	for _, r := range s {
		if r != '#' && r != '.' {
			return false
		}
	}
	return true
}

func (ft *Font) validate() error {
	if len(ft.Order) == 0 {
		return fmt.Errorf("font has no glyphs")
	}
	ft.Height = ft.Glyphs[ft.Order[0]].H
	for _, r := range ft.Order {
		if g := ft.Glyphs[r]; g.H != ft.Height {
			return fmt.Errorf("glyph %q is %d px tall but %q is %d — all glyphs must share one height",
				string(r), g.H, string(ft.Order[0]), ft.Height)
		}
	}
	if ft.LineHeight < 0 {
		ft.LineHeight = ft.Height + 1
	}
	if ft.Baseline < 0 {
		ft.Baseline = ft.Height
	}
	if ft.SpaceWidth < 0 {
		if g, ok := ft.Glyphs['0']; ok {
			ft.SpaceWidth = g.W
		} else {
			ft.SpaceWidth = 3
		}
	}
	if ft.Spacing < 0 {
		return fmt.Errorf("spacing must be >= 0")
	}
	return nil
}

// MaxGlyphWidth returns the widest glyph's width.
func (ft *Font) MaxGlyphWidth() int {
	w := 0
	for _, r := range ft.Order {
		if g := ft.Glyphs[r]; g.W > w {
			w = g.W
		}
	}
	return w
}

// SortedChars returns the glyph chars sorted by codepoint (JSON stability).
func (ft *Font) SortedChars() []rune {
	out := append([]rune(nil), ft.Order...)
	sort.Slice(out, func(i, j int) bool { return out[i] < out[j] })
	return out
}