package hw3test

import (
	"bytes"
	"io"
	"math/rand"
	"testing"
)

func TestComparisonReaderMatchesEnvFile(t *testing.T) {
	for _, seed := range []int64{-7824, 0, 1, 7824, 9223372036854775807} {
		for _, chunkSize := range []int{1, 6, 7, 8, 13, 64 * 1024, 131071} {
			const total = 1024*1024 + 19
			expected := make([]byte, total)
			actual := make([]byte, total)
			_, err := io.ReadFull(rand.New(rand.NewSource(seed)), expected)
			if err != nil {
				t.Fatal(err)
			}
			reader := newComparisonReader(seed)
			for offset := 0; offset < total; {
				end := offset + chunkSize
				if end > total {
					end = total
				}
				if _, err := io.ReadFull(reader, actual[offset:end]); err != nil {
					t.Fatal(err)
				}
				offset = end
			}
			if !bytes.Equal(actual, expected) {
				t.Fatalf("different bytes for seed %d and chunk size %d", seed, chunkSize)
			}
		}
	}
}

func BenchmarkComparisonReaders(b *testing.B) {
	const size = 128 * 1024 * 1024
	buffer := make([]byte, 64*1024)
	for _, candidate := range []struct {
		name string
		open func() io.Reader
	}{
		{"math/rand", func() io.Reader { return rand.New(rand.NewSource(7824)) }},
		{"comparison", func() io.Reader { return newComparisonReader(7824) }},
	} {
		b.Run(candidate.name, func(b *testing.B) {
			b.SetBytes(size)
			for i := 0; i < b.N; i++ {
				reader := candidate.open()
				for remaining := size; remaining > 0; remaining -= len(buffer) {
					if _, err := io.ReadFull(reader, buffer); err != nil {
						b.Fatal(err)
					}
				}
			}
		})
	}
}