// SPDX-License-Identifier: Unlicense

package gigaparsec_test

import (
	"bytes"
	"cmp"
	"io"
	"math"
	"testing"

	"git.codemonkeysoftware.net/b/gigaparsec"
	ptest "git.codemonkeysoftware.net/b/gigaparsec/test"
	"git.codemonkeysoftware.net/b/gigaparsec/test/generator"
	pgen "git.codemonkeysoftware.net/b/gigaparsec/test/generator"
	"github.com/shoenig/test"
	"github.com/shoenig/test/must"
	"pgregory.net/rapid"
)

type customEOFReaderAt struct {
	r             *bytes.Reader
	eofAtExactFit bool
}

func newCustomEOFReaderAt(b []byte, eofAtExactFit bool) customEOFReaderAt {
	return customEOFReaderAt{
		r:             bytes.NewReader(b),
		eofAtExactFit: eofAtExactFit,
	}
}

func (r customEOFReaderAt) ReadAt(p []byte, off int64) (n int, err error) {
	n, err = r.r.ReadAt(p, off)
	if int64(len(p))+off >= r.r.Size() {
		if r.eofAtExactFit {
			err = io.EOF
		} else {
			err = nil
		}
	}
	return n, err
}

func TestState(t *testing.T) {
	t.Run("state reads the same position every time", rapid.MakeCheck(func(t *rapid.T) {
		data := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "data")
		dst := pgen.SliceOfNZero[byte](0, len(data)-1).Draw(t, "dst")
		expected := data[:len(dst)]
		st := gigaparsec.MakeState(bytes.NewReader(data))

		_, next, err := st.Read(dst)
		must.NoError(t, err)
		must.SliceEqOp(t, expected, dst)

		next.Read(dst)
		_, _, err = st.Read(dst)
		must.NoError(t, err)
		must.SliceEqOp(t, expected, dst)
	}))
	t.Run("Read ends before end of source", rapid.MakeCheck(func(t *rapid.T) {
		src := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "src")
		endReadAt := rapid.IntRange(0, len(src)-1).Draw(t, "endReadAt")
		pos := rapid.Uint64Range(0, uint64(endReadAt)).Draw(t, "pos")
		dst := pgen.SliceOfNZero[byte](0, endReadAt-int(pos)).Draw(t, "dst")
		st := gigaparsec.MakeState(bytes.NewReader(src)).At(pos)

		n, next, err := st.At(pos).Read(dst)

		test.EqOp(t, uint64(len(dst)), n)
		ptest.StateIsAt(t, next, pos+n)
		test.NoError(t, err)
		test.SliceEqOp(t, src[pos:pos+n], dst)
	}))
	t.Run("Non-empty Read ends at end of source", rapid.MakeCheck(func(t *rapid.T) {
		readerReturnsEOF := rapid.Bool().Draw(t, "readerReturnsEOF")
		src := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "src")
		dst := pgen.SliceOfNZero[byte](1, len(src)).Draw(t, "dst")
		pos := uint64(len(src) - len(dst))
		st := gigaparsec.MakeState(newCustomEOFReaderAt(src, readerReturnsEOF))

		n, next, err := st.At(pos).Read(dst)

		test.EqOp(t, uint64(len(dst)), n)
		ptest.StateIsAt(t, next, pos+n)
		test.NoError(t, err)
		test.SliceEqOp(t, src[pos:pos+n], dst)
	}))
	t.Run("Read overruns source", rapid.MakeCheck(func(t *rapid.T) {
		src := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "src")
		pos := rapid.Uint64Range(0, uint64(len(src))-1).Draw(t, "pos")
		minDstLen := len(src) - int(pos) + 1
		dst := pgen.SliceOfNZero[byte](minDstLen, minDstLen+10).Draw(t, "dst")
		st := gigaparsec.MakeState(bytes.NewReader(src)).At(pos)

		n, next, err := st.Read(dst)

		test.EqOp(t, uint64(len(src)), n+pos)
		ptest.StateIsAt(t, next, pos+n)
		test.ErrorIs(t, err, io.EOF)
		test.SliceEqOp(t, src[pos:pos+n], dst[:n])
	}))
	t.Run("Read starts after end of source", rapid.MakeCheck(func(t *rapid.T) {
		src := rapid.SliceOfN(rapid.Byte(), 0, 100).Draw(t, "src")
		dst := pgen.SliceOfNZero[byte](0, 100).Draw(t, "dst")
		pos := rapid.Uint64Min(uint64(len(src))).Draw(t, "pos")
		st := gigaparsec.MakeState(bytes.NewReader(src)).At(pos)

		n, next, err := st.Read(dst)

		test.EqOp(t, 0, n)
		ptest.StateIsAt(t, next, pos)
		test.ErrorIs(t, err, io.EOF)
	}))
	t.Run("next state reads next input", rapid.MakeCheck(func(t *rapid.T) {
		const maxLen = 100
		data := rapid.SliceOfN(rapid.Byte(), 1, maxLen).Draw(t, "data")
		skip := rapid.IntRange(0, len(data)-1).Draw(t, "skip")
		st := gigaparsec.MakeState(bytes.NewReader(data))

		_, next, err := st.Read(make([]byte, skip))
		must.NoError(t, err)
		must.EqOp(t, skip, int(next.Pos()))
		dst := make([]byte, maxLen)
		n, _, _ := next.Read(dst)
		must.SliceEqOp(t, data[skip:skip+int(n)], dst[:n])
	}))
	t.Run("At sets state position", rapid.MakeCheck(func(t *rapid.T) {
		data := rapid.SliceOfN(rapid.Byte(), 1, 100).Draw(t, "data")
		pos := rapid.Uint64Range(0, uint64(len(data)-1)).Draw(t, "pos")
		st := gigaparsec.MakeState(bytes.NewReader(data)).At(pos)

		dst := make([]byte, 1)
		n, _, err := st.Read(dst)
		test.EqOp(t, 1, n)
		test.NoError(t, err)
		test.EqOp(t, data[pos], dst[0])
	}))
	t.Run("Pos returns correct position after At", rapid.MakeCheck(func(t *rapid.T) {
		var data []byte
		pos := rapid.Uint64().Draw(t, "pos")
		st := gigaparsec.MakeState(bytes.NewReader(data)).At(pos)
		test.EqOp(t, pos, st.Pos())
	}))
	t.Run("Pos returns correct position after Read", rapid.MakeCheck(func(t *rapid.T) {
		const maxLen = 100
		data := rapid.SliceOfN(rapid.Byte(), 1, maxLen).Draw(t, "data")
		skip := rapid.Uint64Range(0, uint64(len(data)-1)).Draw(t, "skip")
		st := gigaparsec.MakeState(bytes.NewReader(data))

		_, next, err := st.Read(make([]byte, skip))
		must.NoError(t, err)
		test.EqOp(t, skip, next.Pos())
	}))
	t.Run("Read returns an error if the ReaderAt fails", rapid.MakeCheck(func(t *rapid.T) {
		expectedErr := pgen.Error().Draw(t, "expectedErr")
		startPos := rapid.Uint64Max(math.MaxInt64).Draw(t, "startPos")
		dst := pgen.SliceOfNZero[byte](0, 100).Draw(t, "dst")
		st := gigaparsec.MakeState(ptest.ErrReaderAt(expectedErr)).At(startPos)
		n, next, err := st.Read(dst)
		test.ErrorIs(t, err, expectedErr)
		test.EqOp(t, startPos, next.Pos())
		test.Zero(t, n)
	}))
}

func TestSliceReaderAt(t *testing.T) {
	const maxLen = 100
	t.Run("offset ≥ 0", rapid.MakeCheck(func(t *rapid.T) {
		src := rapid.SliceOfN(rapid.Byte(), 0, maxLen).Draw(t, "src")
		dst := generator.SliceOfNZero[byte](0, maxLen).Draw(t, "dst")
		offset := rapid.Int64Range(0, int64(len(src))+10).Draw(t, "offset")

		n, err := gigaparsec.SliceReaderAt[byte](src).ReadAt(dst, offset)

		switch cmp.Compare(len(src), int(offset)+len(dst)) {
		case -1:
			// Read overruns src.
			test.ErrorIs(t, err, io.EOF)
			test.EqOp(t, max(0, len(src)-int(offset)), n)
		case 0:
			// Read exactly reaches end of source.
			// io.ReaderAt spec allows error to be either io.EOF or nil.
			test.EqOp(t, len(dst), n)
		case 1:
			// Read ends before end of source.
			test.NoError(t, err)
			test.EqOp(t, len(dst), n)
		}

		if offset < int64(len(src)) {
			test.SliceEqOp(t, src[offset:offset+int64(n)], dst[:n])
		}
	}))
	t.Run("offset < 0", rapid.MakeCheck(func(t *rapid.T) {
		src := rapid.SliceOfN(rapid.Byte(), 0, maxLen).Draw(t, "src")
		dst := generator.SliceOfNZero[byte](0, maxLen).Draw(t, "dst")
		offset := rapid.Int64Max(-1).Draw(t, "offset")

		n, err := gigaparsec.SliceReaderAt[byte](src).ReadAt(dst, offset)
		test.Error(t, err)
		test.EqOp(t, 0, n)
	}))
}