package main

import (
	iou "0dev.org/ioutil"
	"fmt"
	"io"
	"os"
	"sort"
)

func main() {
	f, err := os.Open(os.Args[1])
	if err != nil {
		os.Stderr.WriteString("Unable to open input file. " + err.Error())
		os.Exit(1)
	}

	pairs, symbols := analyze(f)

	rec := recommend(pairs, symbols)
	fmt.Println(*rec)
}

type recommendation struct {
	p2s map[uint16]byte
	s2p map[byte]uint16
}

func apply(rec *recommendation, reader io.Reader) {
	symbolReader := iou.SizedReader(iou.ReaderFunc(func(output []byte) (int, error) {
		for i := 0; i < len(output)-1; i++ {
			// Read a byte from the underlying reader
			count, err := reader.Read(output[i : i+1])
			if err != nil || count != 1 {
				return i + count, err
			}

			// Convert the byte to a pair if there is a mapping for it
			if pair, ok := rec.s2p[output[i]]; ok {
				output[i] = byte(pair >> 8) // extract the high byte from the pair
				i++
				output[i] = byte(pair) // leave only the low byte from the pair
			}
		}
	}), 2)
}

func recommend(pairs pairSlice, symbols symbolSlice) *recommendation {
	var (
		rec         recommendation
		pairsLength = len(pairs)
	)

	rec.p2s = make(map[uint16]byte) // Store pair to symbol mappings
	rec.s2p = make(map[byte]uint16) // Store symbol to pair mappings

	for i := 0; i < pairsLength; i++ {
		currentPair := pairs[i]

		// Termination condition for when we are out of symbols
		if len(symbols) == 0 {
			break
		}

		gain := currentPair.count - 4 // 4 bytes for the default header
		currentSymbol := symbols[0]

		if currentSymbol.count == 0 {
			// Termination condition for possitive compression effect
			if gain <= 0 {
				break
			}

			// Mark the recommendation and proceed with the next pair
			rec.p2s[currentPair.value] = currentSymbol.value
			continue
		} else { // if the current symbol is present in the data
			// Decrease the gain by a symbol -> p`, p -> symbol replacement
			gain -= 2                   // Additional 2 bytes for the more complex header
			gain -= currentSymbol.count // Account for swaping the symbol to a pair in order to free it

			// Termination condition for possitive compression effect
			if gain <= 0 {
				break
			}

			// Mark this symbol for replacement by the last unused pair
			rec.s2p[currentSymbol.value] = pairs[pairsLength-1].value
			pairsLength--

			// Mark the current pair for replacement by the current symbol
			rec.p2s[currentPair.value] = currentSymbol.value
		}
	}

	return &rec
}

// Reads the provided input and returns information about the available byte pair and used symbols
func analyze(reader io.Reader) (pairSlice, symbolSlice) {
	var (
		current uint16   // Stores a pair of bytes in it's high and low bits
		buffer  []byte   = make([]byte, 1)
		pairs   []uint64 = make([]uint64, 65536) // all possible pairs, 512kb
		symbols []uint64 = make([]uint64, 256)   // all possible characters, 2kb
	)

	// Read the first byte and store in the low bits of the current pair
	if c, err := reader.Read(buffer); err != nil || c != 1 {
		os.Stderr.WriteString("Error reading input.")
		os.Exit(1)
	}
	current = uint16(buffer[0])

	// Read all of the data and note the counts of bytes and byte pairs
	io.Copy(iou.WriterFunc(func(data []byte) (int, error) {
		for _, value := range data {
			// Store pairs frequency
			current <<= 8            // Shift the previous byte from low to high
			current |= uint16(value) // Add the current byte to low
			pairs[current]++

			// Store bytes frequency
			symbols[value]++
		}
		return len(data), nil
	}), reader)

	// Extract and sort all byte pairs
	availablePairs := make(pairSlice, 0)
	for index, value := range pairs {
		availablePairs = append(availablePairs, pair{value: uint16(index), count: value})
	}
	sort.Sort(availablePairs)

	// Extract and sort all symbols (including the ones with zero counts)
	allSymbols := make(symbolSlice, 0)
	for index, value := range symbols {
		allSymbols = append(allSymbols, symbol{value: byte(index), count: value})
	}
	sort.Sort(allSymbols)

	return availablePairs, allSymbols
}

type pair struct {
	value uint16
	count uint64
}

type symbol struct {
	value byte
	count uint64
}

// Implements fmt.Stringer, used for debugging
func (p pair) String() string {
	return fmt.Sprintf("[ %d %d (%d) ]", (p.value >> 8), ((p.value << 8) >> 8), p.count)
}

type pairSlice []pair

func (s pairSlice) Len() int {
	return len(s)
}

func (s pairSlice) Less(i, j int) bool {
	// Sort in descending order
	return s[i].count > s[j].count
}

func (s pairSlice) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}

type symbolSlice []symbol

func (s symbolSlice) Len() int {
	return len(s)
}

func (s symbolSlice) Less(i, j int) bool {
	// Sort in ascending order
	return s[i].count < s[j].count
}

func (s symbolSlice) Swap(i, j int) {
	s[i], s[j] = s[j], s[i]
}