From 6ecfae541c40b92459ecda74f064f4b0ded23666 Mon Sep 17 00:00:00 2001
From: Runxi Yu <me@runxiyu.org>
Date: Thu, 27 Mar 2025 12:02:42 +0800
Subject: Parallel Go solver

---
 challenge.html   |  2 +-
 static/solver.go | 79 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 80 insertions(+), 1 deletion(-)
 create mode 100644 static/solver.go

diff --git a/challenge.html b/challenge.html
index c2f1762..7ff7709 100644
--- a/challenge.html
+++ b/challenge.html
@@ -48,7 +48,7 @@
 		</section>
 
 		<section>
-			<p><a href="/.powxy/static/solver.c">A C implementation</a> and <a href="/.powxy/static/solver.py">a Python implementation</a> of the challenge solver are available.</p>
+			<p>Several offline challenge solvers are available in  <a href="/.powxy/static/solver.c">C</a>, <a href="/.powxy/static/solver.go">Go</a>, and <a href="/.powxy/static/solver.py">Python</a>.
 		</section>
 	</main>
 </body>
diff --git a/static/solver.go b/static/solver.go
new file mode 100644
index 0000000..6aa7835
--- /dev/null
+++ b/static/solver.go
@@ -0,0 +1,79 @@
+// SPDX-License-Identifier: BSD-2-Clause
+// SPDX-FileCopyrightText: Copyright (c) 2025 Runxi Yu <https://runxiyu.org>
+
+package main
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/base64"
+	"encoding/binary"
+	"fmt"
+	"os"
+	"runtime"
+	"strconv"
+)
+
+func validateBitZeros(bs []byte, n int) bool {
+	q := n / 8
+	r := n % 8
+
+	if !bytes.Equal(bs[:q], make([]byte, q)) {
+		return false
+	}
+	if r != 0 && (bs[q]&(0xFF<<(8-r)) != 0) {
+		return false
+	}
+	return true
+}
+
+func main() {
+	if len(os.Args) < 3 {
+		fmt.Fprintln(os.Stderr, "usage: program <base64 input> <difficulty>")
+		os.Exit(1)
+	}
+
+	decoded, err := base64.StdEncoding.DecodeString(os.Args[1])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "invalid base64:", err)
+		os.Exit(1)
+	}
+
+	difficulty, err := strconv.Atoi(os.Args[2])
+	if err != nil {
+		fmt.Fprintln(os.Stderr, "invalid difficulty:", err)
+		os.Exit(1)
+	}
+
+	numWorkers := runtime.NumCPU()
+	result := make(chan uint64)
+
+	for i := 0; i < numWorkers; i++ {
+		go func(start, step int) {
+			var nextVal uint64 = uint64(start)
+
+			buf := append(decoded, make([]byte, 8)...)
+
+			for {
+				binary.LittleEndian.PutUint64(buf[len(buf)-8:], nextVal)
+				h := sha256.Sum256(buf)
+
+				if validateBitZeros(h[:], difficulty) {
+					result <- nextVal
+					return
+				}
+
+				nextVal = (nextVal + uint64(step)) & 0xFFFFFFFFFFFFFFFF
+				if nextVal == uint64(start) {
+					fmt.Fprintln(os.Stderr, "overflow")
+					os.Exit(1)
+				}
+			}
+		}(i, numWorkers)
+	}
+
+	found := <-result
+	out := make([]byte, 8)
+	binary.LittleEndian.PutUint64(out, found)
+	fmt.Println(base64.StdEncoding.EncodeToString(out))
+}
-- 
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