| // Copyright 2017 The Wuffs Authors. |
| // |
| // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or |
| // https://www.apache.org/licenses/LICENSE-2.0> or the MIT license |
| // <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your |
| // option. This file may not be copied, modified, or distributed |
| // except according to those terms. |
| |
| //go:build ignore |
| // +build ignore |
| |
| package main |
| |
| // print-deflate-magic-numbers.go prints the std/deflate lcode_magic_numbers |
| // and dcode_magic_numbers values based on the tables in RFC 1951 secion 3.2.5. |
| // |
| // The lcode base numbers are biased by -3 so that (base_number_minus_3 + |
| // extra_bits) fits in the range [0, 255]. This makes a bitwise-and with 0xFF a |
| // no-op, in terms of computed value, but proves to the compiler that the |
| // result is within a certain range. |
| // |
| // The dcode base numbers are biased by -1 so that (base_number_minus_1 + |
| // extra_bits) fits in the range [0, 32767]. This makes a bitwise-and with |
| // 0x7FFF a no-op, in terms of computed value, but proves to the compiler that |
| // the result is within a certain range. Furthermore, proving that (d + 1) > 0 |
| // is trivial, for d of type u32[..something], compared to proving that d > 0, |
| // which usually requires a runtime check (an if branch). |
| // |
| // Usage: go run print-deflate-magic-numbers.go |
| |
| import ( |
| "fmt" |
| "os" |
| ) |
| |
| func main() { |
| if err := main1(); err != nil { |
| os.Stderr.WriteString(err.Error() + "\n") |
| os.Exit(1) |
| } |
| } |
| |
| func main1() error { |
| // See the "base numbers" comment above. |
| biases := [2]uint32{3, 1} |
| |
| for i := 0; i < 2; i++ { |
| if i != 0 { |
| fmt.Println() |
| } |
| for j := 0; j < 32; j++ { |
| x := uint32(0x08000000) |
| if bn := baseNumbers[i][j]; bn != bad { |
| x = 0x40000000 | ((bn - biases[i]) << 8) | (extraBits[i][j] << 4) |
| } |
| fmt.Printf("0x%08X,", x) |
| if j&7 == 7 { |
| fmt.Println() |
| } |
| } |
| } |
| return nil |
| } |
| |
| const bad = 0xFFFFFFFF |
| |
| var ( |
| baseNumbers = [2][32]uint32{{ |
| 3, 4, 5, 6, 7, 8, 9, 10, |
| 11, 13, 15, 17, 19, 23, 27, 31, |
| 35, 43, 51, 59, 67, 83, 99, 115, |
| 131, 163, 195, 227, 258, bad, bad, bad, |
| }, { |
| 1, 2, 3, 4, 5, 7, 9, 13, |
| 17, 25, 33, 49, 65, 97, 129, 193, |
| 257, 385, 513, 769, 1025, 1537, 2049, 3073, |
| 4097, 6145, 8193, 12289, 16385, 24577, bad, bad, |
| }} |
| |
| extraBits = [2][32]uint32{{ |
| 0, 0, 0, 0, 0, 0, 0, 0, |
| 1, 1, 1, 1, 2, 2, 2, 2, |
| 3, 3, 3, 3, 4, 4, 4, 4, |
| 5, 5, 5, 5, 0, bad, bad, bad, |
| }, { |
| 0, 0, 0, 0, 1, 1, 2, 2, |
| 3, 3, 4, 4, 5, 5, 6, 6, |
| 7, 7, 8, 8, 9, 9, 10, 10, |
| 11, 11, 12, 12, 13, 13, bad, bad, |
| }} |
| ) |