Initial commit

Author: MartinodF

.gitignore

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+a.out*
+primes
+primes.exe
+primes.txt

LICENSE

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+Copyright (c) 2012 Martino di Filippo <[email protected]>
+
+Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

README.md

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+Primes.go
+====
+
+### Introduction
+
+A quick test implementing the Sieve of Eratostenes in Go.
+
+It searches for prime numbers up to the specified limit, optionally saving them to file.
+
+It is one of many small programs I wrote to get a feel for [#golang](http://golang.org/). If you're interested in generating prime numbers, you should probably look into [primegen](http://cr.yp.to/primegen.html) or some other, more suitable alternative.
+
+### Compiling
+
+    $ go build primes.go
+
+### Usage
+
+primes [-l limit] [-s]
+
+  * -l limit *(default 100000)* Upper limit for prime searching
+  * -s *(default false)* Save the prime numbers to primes.txt
+
+### Speed
+
+These are some non-accurate benchmarks I ran on my machine.
+
+**Specs**
+
+* AMD Phenom II X6 1055T @3.4GHz
+* 8GB of cheap Kingston DDR3 @486MHz
+* Windows 8 RTM 64-bit
+
+**Results**
+
+Ram usage and CPU time for finding primes up to:
+
+           1'000'000    <1MB 2ms
+          10'000'000    <1MB 26ms
+         100'000'000   7.4MB 401ms
+       1'000'000'000  61.2MB 9.01s
+      10'000'000'000 599.8MB 1m51s
+
+### License
+
+Copyright 2012 Martino di Filippo
+
+Licensed under the MIT License

primes.go

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+package main
+
+import (
+  "flag"
+  "fmt"
+  "os"
+  "path/filepath"
+  "time"
+)
+
+func allocate(limit int64) []uint64 {
+  bits := uint64((limit / 2) - 1)
+  mod  := bits % 64
+  ints := (bits - mod) / 64 + 1
+
+  primes := make([]uint64, ints)
+
+  if mod != 0 {
+    // if the last bitmask isn't complete, set all remaining bits to 1
+    primes[ints - 1] |= (0xFFFFFFFFFFFFFFFF << mod)
+  }
+
+  return primes
+}
+
+func sieve(limit uint64, primes []uint64) {
+  fmt.Printf("Finding all primes <= %v\n", limit)
+
+  start := time.Now()
+
+  for k, i, run := uint64(0), uint64(3), true; run; k++ {
+    for l := uint8(0); l < 64; l, i = l + 1, i + 2 {
+      if (primes[k] >> l) & 1 == 1 {
+        // number was already marked as composite
+        continue
+      }
+
+      sqr := i * i
+      if sqr > limit {
+        run = false
+        break
+      }
+
+      for d := i * 2; sqr <= limit; sqr += d {
+        // mark all odd multiples from i*i to limit as composites
+        p := uint64((sqr - 3) / 2)
+        primes[p / 64] |= (1 << (p % 64))
+      }
+    }
+  }
+
+  fmt.Printf("Done in %v.\n", time.Since(start))
+}
+
+func save(primes []uint64) {
+  fmt.Printf("\nSaving to primes.txt...\n");
+  ints := uint(len(primes))
+  start := time.Now()
+
+  file, err := os.Create("primes.txt")
+  if err != nil { panic(err) }
+  defer file.Close()
+
+  fmt.Fprintln(file, 2)
+  for k := uint(0); k < ints; k++ {
+    for l := uint(0); l < 64; l++ {
+      if (primes[k] >> l) & 1 == 0 {
+        fmt.Fprintln(file, (k * 64 + l) * 2 + 3)
+      }
+    }
+  }
+
+  fmt.Printf("Done in %v.\n", time.Since(start))
+}
+
+func main() {
+  limit := flag.Int64("l", 100000, "Upper limit for prime searching")
+  write := flag.Bool("s", false, "Save the prime numbers to primes.txt")
+
+  flag.Usage = func() {
+    fmt.Fprintln(os.Stderr, "A quick test implementing the Sieve of Eratostenes in Go")
+    fmt.Fprintln(os.Stderr, "It searches for prime numbers up to the specified limit\n")
+    fmt.Fprintf(os.Stderr, "Usage: %s [-l limit] [-s]\n\n", filepath.Base(os.Args[0]))
+    flag.PrintDefaults()
+    fmt.Fprint(os.Stderr, "\n")
+  }
+
+  flag.Parse()
+
+  primes := allocate(*limit)
+  sieve(uint64(*limit), primes)
+
+  if *write {
+    save(primes)
+  }
+}