adler32() をハッシュ関数として使用する場合、まれに衝突が発生することが予想されます。
衝突の確率を正確に計算することはできますが、大まかに言えば、
これは 32 ビットのハッシュ関数である
ため、数千のアイテムのサンプル セットで多くの衝突が発生することはありません。
これはほとんど当てはまりません。
以下に例を示します: 真ん中に日付を含む文字列を考えてみましょう。
"Some prefix text " + date + " some postfix text."
ここで、日付の形式は yyyy-mm-dd で、2012 年をループします。
この例では 91 回の衝突があります。
さらに悪いことに、3 つの日付が重なったケースが 7 つあります。
このように一般的に使用されるハッシュ関数のパフォーマンスが低いのはなぜでしょうか?
または、何か不足していますか？
上記の例の詳細な結果は次のとおりです。
0x592a0f1f: 2012-01-30, 2012-02-02, 2012-10-21
0x592b0f1f: 2012-02-11, 2012-10-30, 2012-11-02
0x593d0f20: 2012-01-31, 2012-02-03, 2012-10-22
0x593e0f20: 2012-02-12, 2012-10-31, 2012-11-03
0x59410f20: 2012-03-11, 2012-11-30, 2012-12-02
0x59560f21: 2012-03-30, 2012-04-02, 2012-12-21
0x59690f22: 2012-03-31, 2012-04-03, 2012-12-22
0x59020f1d: 2012-01-10, 2012-10-01
0x59150f1e: 2012-01-11, 2012-10-02
0x59160f1e: 2012-01-20, 2012-10-11
0x59170f1e: 2012-02-01, 2012-10-20
0x59180f1e: 2012-02-10, 2012-11-01
0x59280f1f: 2012-01-12, 2012-10-03
0x59290f1f: 2012-01-21, 2012-10-12
0x592c0f1f: 2012-02-20, 2012-11-11
0x592d0f1f: 2012-03-01, 2012-11-20
0x592e0f1f: 2012-03-10, 2012-12-01
0x593b0f20: 2012-01-13, 2012-10-04
0x593c0f20: 2012-01-22, 2012-10-13
0x593f0f20: 2012-02-21, 2012-11-12
0x59400f20: 2012-03-02, 2012-11-21
0x59420f20: 2012-03-20, 2012-12-11
0x59430f20: 2012-04-01, 2012-12-20
0x594e0f21: 2012-01-14, 2012-10-05
0x594f0f21: 2012-01-23, 2012-10-14
0x59500f21: 2012-02-04, 2012-10-23
0x59510f21: 2012-02-13, 2012-11-04
0x59520f21: 2012-02-22, 2012-11-13
0x59530f21: 2012-03-03, 2012-11-22
0x59540f21: 2012-03-12, 2012-12-03
0x59550f21: 2012-03-21, 2012-12-12
0x59570f21: 2012-04-11, 2012-12-30
0x59610f22: 2012-01-15, 2012-10-06
0x59620f22: 2012-01-24, 2012-10-15
0x59630f22: 2012-02-05, 2012-10-24
0x59640f22: 2012-02-14, 2012-11-05
0x59650f22: 2012-02-23, 2012-11-14
0x59660f22: 2012-03-04, 2012-11-23
0x59670f22: 2012-03-13, 2012-12-04
0x59680f22: 2012-03-22, 2012-12-13
0x596a0f22: 2012-04-12, 2012-12-31
0x596c0f22: 2012-04-30, 2012-05-02
0x59740f23: 2012-01-16, 2012-10-07
0x59750f23: 2012-01-25, 2012-10-16
0x59760f23: 2012-02-06, 2012-10-25
0x59770f23: 2012-02-15, 2012-11-06
0x59780f23: 2012-02-24, 2012-11-15
0x59790f23: 2012-03-05, 2012-11-24
0x597a0f23: 2012-03-14, 2012-12-05
0x597b0f23: 2012-03-23, 2012-12-14
0x597c0f23: 2012-04-04, 2012-12-23
0x59820f23: 2012-05-30, 2012-06-02
0x59870f24: 2012-01-17, 2012-10-08
0x59880f24: 2012-01-26, 2012-10-17
0x59890f24: 2012-02-07, 2012-10-26
0x598a0f24: 2012-02-16, 2012-11-07
0x598b0f24: 2012-02-25, 2012-11-16
0x598c0f24: 2012-03-06, 2012-11-25
0x598d0f24: 2012-03-15, 2012-12-06
0x598e0f24: 2012-03-24, 2012-12-15
0x598f0f24: 2012-04-05, 2012-12-24
0x59950f24: 2012-05-31, 2012-06-03
0x59980f24: 2012-06-30, 2012-07-02
0x599a0f25: 2012-01-18, 2012-10-09
0x599b0f25: 2012-01-27, 2012-10-18
0x599c0f25: 2012-02-08, 2012-10-27
0x599d0f25: 2012-02-17, 2012-11-08
0x599e0f25: 2012-02-26, 2012-11-17
0x599f0f25: 2012-03-07, 2012-11-26
0x59a00f25: 2012-03-16, 2012-12-07
0x59a10f25: 2012-03-25, 2012-12-16
0x59a20f25: 2012-04-06, 2012-12-25
0x59ae0f25: 2012-07-30, 2012-08-02
0x59ae0f26: 2012-01-28, 2012-10-19
0x59af0f26: 2012-02-09, 2012-10-28
0x59b00f26: 2012-02-18, 2012-11-09
0x59b10f26: 2012-02-27, 2012-11-18
0x59b20f26: 2012-03-08, 2012-11-27
0x59b30f26: 2012-03-17, 2012-12-08
0x59b40f26: 2012-03-26, 2012-12-17
0x59b50f26: 2012-04-07, 2012-12-26
0x59c10f26: 2012-07-31, 2012-08-03
0x59c40f26: 2012-08-30, 2012-09-02
0x59c40f27: 2012-02-28, 2012-11-19
0x59c50f27: 2012-03-09, 2012-11-28
0x59c60f27: 2012-03-18, 2012-12-09
0x59c70f27: 2012-03-27, 2012-12-18
0x59c80f27: 2012-04-08, 2012-12-27
0x59d70f27: 2012-08-31, 2012-09-03
0x59da0f28: 2012-03-28, 2012-12-19
0x59db0f28: 2012-04-09, 2012-12-28