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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(pairs)
fmt.Println(symbols)
fmt.Println(*rec)
}
type pair struct {
value uint16
count uint64
}
type symbol struct {
value byte
count uint64
}
type recommendation struct {
p2s map[uint16]byte
s2p map[byte]uint16
}
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)
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// Store bytes frequency
symbols[value]++
}
return len(data), nil
}), reader)
// Extract and sort all available byte pairs
// Extract and sort all byte pairs
availablePairs := make(pairSlice, 0)
for index, value := range pairs {
if value > 0 {
availablePairs = append(availablePairs, pair{value: uint16(index), count: value})
}
type pair struct {
value uint16
count uint64
}
type symbol struct {
value byte
count uint64
}
// Sort in descending order
return s[i].count > s[j].count
// 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]
}
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