implement basic hashcash-like proof of work

go.mod

@@ -0,0 +1,3 @@
+module charon
+
+go 1.26

main.go

@@ -0,0 +1,107 @@
+package main
+
+import (
+	"crypto"
+	_ "crypto/sha1"
+	"encoding/hex"
+	"fmt"
+	"math/rand"
+	"time"
+)
+
+func main() {
+	g := Generator{
+		Difficulty: 26,
+	}
+	c := g.RandomChallenge()
+	fmt.Println(c.String())
+	fmt.Println(c.Solve().String())
+}
+
+type Challenge struct {
+	Input      [48]byte
+	Date       [20]byte
+	Difficulty byte
+	Solution   int64
+}
+type Generator struct {
+	Difficulty byte
+}
+
+func (g *Generator) RandomChallenge() Challenge {
+	bytes := make([]byte, 48)
+	for i, _ := range bytes {
+		bytes[i] = byte(rand.Int())
+	}
+	today := time.Now().UTC()
+	date := today.Format("2006-01-02")
+	hash := crypto.SHA1.New().Sum([]byte(date))
+	return Challenge{
+		Input:      [48]byte(bytes),
+		Date:       [20]byte(hash),
+		Difficulty: g.Difficulty,
+	}
+}
+
+// using a lookup table for this is super goofy but i'm lazy and it's 6am
+var prefixLookup = []int{
+	8, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
+	3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
+	2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+	2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2,
+	1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+	1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+	1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+	1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+	0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+}
+
+func (c Challenge) Solve() Challenge {
+	bytes := make([]byte, 76)
+	bytes = append(bytes, c.Input[:]...)
+	bytes = append(bytes, c.Date[:]...)
+	for {
+		for i := range 8 {
+			bytes[75-i] = byte(c.Solution >> (i * 8))
+		}
+		sum := make([]byte, 20)
+		hash := crypto.SHA1.New()
+		hash.Write(bytes)
+		sum = hash.Sum(nil)
+		count := 0
+		for _, c := range sum {
+			p := prefixLookup[c]
+			count += p
+			if p != 8 {
+				break
+			}
+		}
+		if c.Solution%20000000 == 0 {
+			println(c.Solution)
+		}
+		if count >= int(c.Difficulty) {
+			println(c.Solution)
+			break
+		}
+		c.Solution++
+	}
+	return c
+}
+func (c Challenge) String() string {
+	bytes := make([]byte, 0)
+	bytes = append(bytes, c.Input[:]...)
+	bytes = append(bytes, c.Date[:]...)
+	bytes = append(bytes, make([]byte, 8)...)
+	bytes[68] = c.Difficulty
+	for i := range 7 {
+		bytes[75-i] = byte(c.Solution >> (i * 8))
+	}
+	return hex.EncodeToString(bytes)
+}