package question

import (
	"encoding/binary"
	"testing"

	"github.com/stretchr/testify/require"
)

// buildTTC 拼一个最小的 TrueType Collection：两个字面共用一套表，
// 表数据刻意按「与目录相反」的顺序摆放，用来证明提取逻辑是按目录里的 offset
// 搬数据，而不是简单地顺序切片。
func buildTTC(t *testing.T) []byte {
	t.Helper()

	// 表内容
	headData := []byte{0xDE, 0xAD, 0xBE, 0xEF}
	cmapData := []byte{0x01, 0x02, 0x03, 0x04, 0x05, 0x06}

	const ttcHeaderLen = 16
	const sfntHeaderLen = 12
	const recLen = 16
	const numTables = 2
	sfntStart := ttcHeaderLen
	dirStart := sfntStart + sfntHeaderLen
	dataStart := dirStart + recLen*numTables
	// 先放 cmap 再放 head（与目录顺序相反）
	cmapOff := dataStart
	headOff := cmapOff + len(cmapData)

	buf := make([]byte, headOff+len(headData))

	copy(buf[0:4], "ttcf")
	binary.BigEndian.PutUint32(buf[4:8], 0x00010000) // version
	binary.BigEndian.PutUint32(buf[8:12], 1)         // numFonts
	binary.BigEndian.PutUint32(buf[12:16], uint32(sfntStart))

	binary.BigEndian.PutUint32(buf[sfntStart:sfntStart+4], 0x00010000) // sfntVersion
	binary.BigEndian.PutUint16(buf[sfntStart+4:sfntStart+6], numTables)
	binary.BigEndian.PutUint16(buf[sfntStart+6:sfntStart+8], 16) // searchRange（随便填，提取时应重算）
	binary.BigEndian.PutUint16(buf[sfntStart+8:sfntStart+10], 0)
	binary.BigEndian.PutUint16(buf[sfntStart+10:sfntStart+12], 0)

	// 第 1 条记录：head → 数据在末尾
	p := dirStart
	copy(buf[p:p+4], "head")
	binary.BigEndian.PutUint32(buf[p+4:p+8], 0x11111111) // checksum
	binary.BigEndian.PutUint32(buf[p+8:p+12], uint32(headOff))
	binary.BigEndian.PutUint32(buf[p+12:p+16], uint32(len(headData)))

	// 第 2 条记录：cmap → 数据在前
	p = dirStart + recLen
	copy(buf[p:p+4], "cmap")
	binary.BigEndian.PutUint32(buf[p+4:p+8], 0x22222222)
	binary.BigEndian.PutUint32(buf[p+8:p+12], uint32(cmapOff))
	binary.BigEndian.PutUint32(buf[p+12:p+16], uint32(len(cmapData)))

	copy(buf[cmapOff:], cmapData)
	copy(buf[headOff:], headData)
	return buf
}

func TestTTCFirstFont(t *testing.T) {
	ttc := buildTTC(t)
	out, err := ttcFirstFont(ttc)
	require.NoError(t, err)

	// sfnt header
	require.Equal(t, uint32(0x00010000), binary.BigEndian.Uint32(out[0:4]))
	require.Equal(t, uint16(2), binary.BigEndian.Uint16(out[4:6]))
	// searchRange = 最大 2 的幂 × 16 = 2 × 16 = 32；entrySelector = 1；rangeShift = 32-32 = 0
	require.Equal(t, uint16(32), binary.BigEndian.Uint16(out[6:8]))
	require.Equal(t, uint16(1), binary.BigEndian.Uint16(out[8:10]))
	require.Equal(t, uint16(0), binary.BigEndian.Uint16(out[10:12]))

	// 目录：tag / checksum / length 原样保留，offset 指向新位置
	type rec struct {
		tag      string
		checksum uint32
		off, ln  uint32
	}
	var recs []rec
	for i := 0; i < 2; i++ {
		p := 12 + 16*i
		recs = append(recs, rec{
			tag:      string(out[p : p+4]),
			checksum: binary.BigEndian.Uint32(out[p+4 : p+8]),
			off:      binary.BigEndian.Uint32(out[p+8 : p+12]),
			ln:       binary.BigEndian.Uint32(out[p+12 : p+16]),
		})
	}
	require.Equal(t, "head", recs[0].tag)
	require.Equal(t, "cmap", recs[1].tag)
	require.Equal(t, uint32(0x11111111), recs[0].checksum)
	require.Equal(t, uint32(0x22222222), recs[1].checksum)

	// 表数据按新 offset 读出来必须与原始一致
	require.Equal(t, []byte{0xDE, 0xAD, 0xBE, 0xEF},
		out[recs[0].off:recs[0].off+recs[0].ln])
	require.Equal(t, []byte{0x01, 0x02, 0x03, 0x04, 0x05, 0x06},
		out[recs[1].off:recs[1].off+recs[1].ln])

	// 每张表在新文件里都要 4 字节对齐（sfnt 规范要求）
	require.Zero(t, recs[0].off%4)
	require.Zero(t, recs[1].off%4)
}

func TestTTCFirstFontRejects(t *testing.T) {
	cases := map[string][]byte{
		"空数据":    nil,
		"不是 TTC": []byte("not a font file at all"),
		"头部被截断":  []byte("ttcf\x00\x01\x00\x00"),
		"numFonts 为 0": func() []byte {
			b := make([]byte, 16)
			copy(b, "ttcf")
			binary.BigEndian.PutUint32(b[4:8], 0x00010000)
			return b
		}(),
		"子字体偏移越界": func() []byte {
			b := make([]byte, 16)
			copy(b, "ttcf")
			binary.BigEndian.PutUint32(b[4:8], 0x00010000)
			binary.BigEndian.PutUint32(b[8:12], 1)
			binary.BigEndian.PutUint32(b[12:16], 4096)
			return b
		}(),
	}
	for name, data := range cases {
		t.Run(name, func(t *testing.T) {
			_, err := ttcFirstFont(data)
			require.Error(t, err)
		})
	}
}

// TestTTCFirstFontRejectsBadTableOffset 表目录里的 offset 指向文件外时必须报错，
// 而不是把越界切片交给上层（否则就是一次 panic 而不是 500）。
func TestTTCFirstFontRejectsBadTableOffset(t *testing.T) {
	ttc := buildTTC(t)
	// 第 1 条记录位于 ttcHeader(16) + sfntHeader(12)，其 length 字段在记录内第 12 字节
	p := 16 + 12 + 12
	binary.BigEndian.PutUint32(ttc[p:p+4], 1<<20)

	_, err := ttcFirstFont(ttc)
	require.Error(t, err)
}