Add Bech32 error location function

1 files changed, 311 insertions, 8 deletions

diff --git a/src/bech32.cpp b/src/bech32.cpp
index 9da2488ef2..905a2aa2c0 100644
--- a/src/bech32.cpp
+++ b/src/bech32.cpp
@@ -30,6 +30,155 @@ const int8_t CHARSET_REV[128] = {
      1,  0,  3, 16, 11, 28, 12, 14,  6,  4,  2, -1, -1, -1, -1, -1
 };
 
+
+const int16_t GF1024_EXP[] = {
+    1, 303, 635, 446, 997, 640, 121, 142, 959, 420, 350, 438, 166, 39, 543,
+    335, 831, 691, 117, 632, 719, 97, 107, 374, 558, 797, 54, 150, 858, 877,
+    724, 1013, 294, 23, 354, 61, 164, 633, 992, 538, 469, 659, 174, 868, 184,
+    809, 766, 563, 866, 851, 257, 520, 45, 770, 535, 524, 408, 213, 436, 760,
+    472, 330, 933, 799, 616, 361, 15, 391, 756, 814, 58, 608, 554, 680, 993,
+    821, 942, 813, 843, 484, 193, 935, 321, 919, 572, 741, 423, 559, 562,
+    589, 296, 191, 493, 685, 891, 665, 435, 60, 395, 2, 606, 511, 853, 746,
+    32, 219, 284, 631, 840, 661, 837, 332, 78, 311, 670, 887, 111, 195, 505,
+    190, 194, 214, 709, 380, 819, 69, 261, 957, 1018, 161, 739, 588, 7, 708,
+    83, 328, 507, 736, 317, 899, 47, 348, 1000, 345, 882, 245, 367, 996, 943,
+    514, 304, 90, 804, 295, 312, 793, 387, 833, 249, 921, 660, 618, 823, 496,
+    722, 30, 782, 225, 892, 93, 480, 372, 112, 738, 867, 636, 890, 950, 968,
+    386, 622, 642, 551, 369, 234, 846, 382, 365, 442, 592, 343, 986, 122,
+    1023, 59, 847, 81, 790, 4, 437, 983, 931, 244, 64, 415, 529, 487, 944,
+    35, 938, 664, 156, 583, 53, 999, 222, 390, 987, 341, 388, 389, 170, 721,
+    879, 138, 522, 627, 765, 322, 230, 440, 14, 168, 143, 656, 991, 224, 595,
+    550, 94, 657, 752, 667, 1005, 451, 734, 744, 638, 292, 585, 157, 872,
+    590, 601, 827, 774, 930, 475, 571, 33, 500, 871, 969, 173, 21, 828, 450,
+    1009, 147, 960, 705, 201, 228, 998, 497, 1021, 613, 688, 772, 508, 36,
+    366, 715, 468, 956, 725, 730, 861, 425, 647, 701, 221, 759, 95, 958, 139,
+    805, 8, 835, 679, 614, 449, 128, 791, 299, 974, 617, 70, 628, 57, 273,
+    430, 67, 750, 405, 780, 703, 643, 776, 778, 340, 171, 1022, 276, 308,
+    495, 243, 644, 460, 857, 28, 336, 286, 41, 695, 448, 431, 364, 149, 43,
+    233, 63, 762, 902, 181, 240, 501, 584, 434, 275, 1008, 444, 443, 895,
+    812, 612, 927, 383, 66, 961, 1006, 690, 346, 3, 881, 900, 747, 271, 672,
+    162, 402, 456, 748, 971, 755, 490, 105, 808, 977, 72, 732, 182, 897, 625,
+    163, 189, 947, 850, 46, 115, 403, 231, 151, 629, 278, 874, 16, 934, 110,
+    492, 898, 256, 807, 598, 700, 498, 140, 481, 91, 523, 860, 134, 252, 771,
+    824, 119, 38, 816, 820, 641, 342, 757, 513, 577, 990, 463, 40, 920, 955,
+    17, 649, 533, 82, 103, 896, 862, 728, 259, 86, 466, 87, 253, 556, 323,
+    457, 963, 432, 845, 527, 745, 849, 863, 1015, 888, 488, 567, 727, 132,
+    674, 764, 109, 669, 6, 1003, 552, 246, 542, 96, 324, 781, 912, 248, 694,
+    239, 980, 210, 880, 683, 144, 177, 325, 546, 491, 326, 339, 623, 941, 92,
+    207, 783, 462, 263, 483, 517, 1012, 9, 620, 220, 984, 548, 512, 878, 421,
+    113, 973, 280, 962, 159, 310, 945, 268, 465, 806, 889, 199, 76, 873, 865,
+    34, 645, 227, 290, 418, 693, 926, 80, 569, 639, 11, 50, 291, 141, 206,
+    544, 949, 185, 518, 133, 909, 135, 467, 376, 646, 914, 678, 841, 954,
+    318, 242, 939, 951, 743, 1017, 976, 359, 167, 264, 100, 241, 218, 51, 12,
+    758, 368, 453, 309, 192, 648, 826, 553, 473, 101, 478, 673, 397, 1001,
+    118, 265, 331, 650, 356, 982, 652, 655, 510, 634, 145, 414, 830, 924,
+    526, 966, 298, 737, 18, 504, 401, 697, 360, 288, 1020, 842, 203, 698,
+    537, 676, 279, 581, 619, 536, 907, 876, 1019, 398, 152, 1010, 994, 68,
+    42, 454, 580, 836, 99, 565, 137, 379, 503, 22, 77, 582, 282, 412, 352,
+    611, 347, 300, 266, 570, 270, 911, 729, 44, 557, 108, 946, 637, 597, 461,
+    630, 615, 238, 763, 681, 718, 334, 528, 200, 459, 413, 79, 24, 229, 713,
+    906, 579, 384, 48, 893, 370, 923, 202, 917, 98, 794, 754, 197, 530, 662,
+    52, 712, 677, 56, 62, 981, 509, 267, 789, 885, 561, 316, 684, 596, 226,
+    13, 985, 779, 123, 720, 576, 753, 948, 406, 125, 315, 104, 519, 426, 502,
+    313, 566, 1016, 767, 796, 281, 749, 740, 136, 84, 908, 424, 936, 198,
+    355, 274, 735, 967, 5, 154, 428, 541, 785, 704, 486, 671, 600, 532, 381,
+    540, 574, 187, 88, 378, 216, 621, 499, 419, 922, 485, 494, 476, 255, 114,
+    188, 668, 297, 400, 918, 787, 158, 25, 458, 178, 564, 422, 768, 73, 1011,
+    717, 575, 404, 547, 196, 829, 237, 394, 301, 37, 65, 176, 106, 89, 85,
+    675, 979, 534, 803, 995, 363, 593, 120, 417, 452, 26, 699, 822, 223, 169,
+    416, 235, 609, 773, 211, 607, 208, 302, 852, 965, 603, 357, 761, 247,
+    817, 539, 250, 232, 272, 129, 568, 848, 624, 396, 710, 525, 183, 686, 10,
+    285, 856, 307, 811, 160, 972, 55, 441, 289, 723, 305, 373, 351, 153, 733,
+    409, 506, 975, 838, 573, 970, 988, 913, 471, 205, 337, 49, 594, 777, 549,
+    815, 277, 27, 916, 333, 353, 844, 800, 146, 751, 186, 375, 769, 358, 392,
+    883, 474, 788, 602, 74, 130, 329, 212, 155, 131, 102, 687, 293, 870, 742,
+    726, 427, 217, 834, 904, 29, 127, 869, 407, 338, 832, 470, 482, 810, 399,
+    439, 393, 604, 929, 682, 447, 714, 251, 455, 875, 319, 477, 464, 521,
+    258, 377, 937, 489, 792, 172, 314, 327, 124, 20, 531, 953, 591, 886, 320,
+    696, 71, 859, 578, 175, 587, 707, 663, 283, 179, 795, 989, 702, 940, 371,
+    692, 689, 555, 903, 410, 651, 75, 429, 818, 362, 894, 515, 31, 545, 666,
+    706, 952, 864, 269, 254, 349, 711, 802, 716, 784, 1007, 925, 801, 445,
+    148, 260, 658, 385, 287, 262, 204, 126, 586, 1004, 236, 165, 854, 411,
+    932, 560, 19, 215, 1002, 775, 653, 928, 901, 964, 884, 798, 839, 786,
+    433, 610, 116, 855, 180, 479, 910, 1014, 599, 915, 905, 306, 516, 731,
+    626, 978, 825, 344, 605, 654, 209
+};
+static_assert(std::size(GF1024_EXP) == 1023, "GF1024_EXP length should be 1023");
+
+const int16_t GF1024_LOG[] = {
+    -1, 0, 99, 363, 198, 726, 462, 132, 297, 495, 825, 528, 561, 693, 231,
+    66, 396, 429, 594, 990, 924, 264, 627, 33, 660, 759, 792, 858, 330, 891,
+    165, 957, 104, 259, 518, 208, 280, 776, 416, 13, 426, 333, 618, 339, 641,
+    52, 388, 140, 666, 852, 529, 560, 678, 213, 26, 832, 681, 309, 70, 194,
+    97, 35, 682, 341, 203, 777, 358, 312, 617, 125, 307, 931, 379, 765, 875,
+    951, 515, 628, 112, 659, 525, 196, 432, 134, 717, 781, 438, 440, 740,
+    780, 151, 408, 487, 169, 239, 293, 467, 21, 672, 622, 557, 571, 881, 433,
+    704, 376, 779, 22, 643, 460, 398, 116, 172, 503, 751, 389, 1004, 18, 576,
+    415, 789, 6, 192, 696, 923, 702, 981, 892, 302, 816, 876, 880, 457, 537,
+    411, 539, 716, 624, 224, 295, 406, 531, 7, 233, 478, 586, 864, 268, 974,
+    338, 27, 392, 614, 839, 727, 879, 211, 250, 758, 507, 830, 129, 369, 384,
+    36, 985, 12, 555, 232, 796, 221, 321, 920, 263, 42, 934, 778, 479, 761,
+    939, 1006, 344, 381, 823, 44, 535, 866, 739, 752, 385, 119, 91, 566, 80,
+    120, 117, 771, 675, 721, 514, 656, 271, 670, 602, 980, 850, 532, 488,
+    803, 1022, 475, 801, 878, 57, 121, 991, 742, 888, 559, 105, 497, 291,
+    215, 795, 236, 167, 692, 520, 272, 661, 229, 391, 814, 340, 184, 798,
+    984, 773, 650, 473, 345, 558, 548, 326, 202, 145, 465, 810, 471, 158,
+    813, 908, 412, 441, 964, 750, 401, 50, 915, 437, 975, 126, 979, 491, 556,
+    577, 636, 685, 510, 963, 638, 367, 815, 310, 723, 349, 323, 857, 394,
+    606, 505, 713, 630, 938, 106, 826, 332, 978, 599, 834, 521, 530, 248,
+    883, 32, 153, 90, 754, 592, 304, 635, 775, 804, 1, 150, 836, 1013, 828,
+    324, 565, 508, 113, 154, 708, 921, 703, 689, 138, 547, 911, 929, 82, 228,
+    443, 468, 480, 483, 922, 135, 877, 61, 578, 111, 860, 654, 15, 331, 851,
+    895, 484, 320, 218, 420, 190, 1019, 143, 362, 634, 141, 965, 10, 838,
+    632, 861, 34, 722, 580, 808, 869, 554, 598, 65, 954, 787, 337, 187, 281,
+    146, 563, 183, 668, 944, 171, 837, 23, 867, 541, 916, 741, 625, 123, 736,
+    186, 357, 665, 977, 179, 156, 219, 220, 216, 67, 870, 902, 774, 98, 820,
+    574, 613, 900, 755, 596, 370, 390, 769, 314, 701, 894, 56, 841, 949, 987,
+    631, 658, 587, 204, 797, 790, 522, 745, 9, 502, 763, 86, 719, 288, 706,
+    887, 728, 952, 311, 336, 446, 1002, 348, 96, 58, 199, 11, 901, 230, 833,
+    188, 352, 351, 973, 3, 906, 335, 301, 266, 244, 791, 564, 619, 909, 371,
+    444, 760, 657, 328, 647, 490, 425, 913, 511, 439, 540, 283, 40, 897, 849,
+    60, 570, 872, 257, 749, 912, 572, 1007, 170, 407, 898, 492, 79, 747, 732,
+    206, 454, 918, 375, 482, 399, 92, 748, 325, 163, 274, 405, 744, 260, 346,
+    707, 626, 595, 118, 842, 136, 279, 684, 584, 101, 500, 422, 149, 956,
+    1014, 493, 536, 705, 51, 914, 225, 409, 55, 822, 590, 448, 655, 205, 676,
+    925, 735, 431, 784, 54, 609, 604, 39, 812, 737, 729, 466, 14, 533, 958,
+    481, 770, 499, 855, 238, 182, 464, 569, 72, 947, 442, 642, 24, 87, 989,
+    688, 88, 47, 762, 623, 709, 455, 817, 526, 637, 258, 84, 845, 738, 768,
+    698, 423, 933, 664, 620, 607, 629, 212, 347, 249, 982, 935, 131, 89, 252,
+    927, 189, 788, 853, 237, 691, 646, 403, 1010, 734, 253, 874, 807, 903,
+    1020, 100, 802, 71, 799, 1003, 633, 355, 276, 300, 649, 64, 306, 161,
+    608, 496, 743, 180, 485, 819, 383, 1016, 226, 308, 393, 648, 107, 19, 37,
+    585, 2, 175, 645, 247, 527, 5, 419, 181, 317, 327, 519, 542, 289, 567,
+    430, 579, 950, 582, 994, 1021, 583, 234, 240, 976, 41, 160, 109, 677,
+    937, 210, 95, 959, 242, 753, 461, 114, 733, 368, 573, 458, 782, 605, 680,
+    544, 299, 73, 652, 905, 477, 690, 93, 824, 882, 277, 946, 361, 17, 945,
+    523, 472, 334, 930, 597, 603, 793, 404, 290, 942, 316, 731, 270, 960,
+    936, 133, 122, 821, 966, 679, 662, 907, 282, 968, 767, 653, 20, 697, 222,
+    164, 835, 30, 285, 886, 456, 436, 640, 286, 1015, 380, 840, 245, 724,
+    137, 593, 173, 130, 715, 85, 885, 551, 246, 449, 103, 366, 372, 714, 313,
+    865, 241, 699, 674, 374, 68, 421, 562, 292, 59, 809, 342, 651, 459, 227,
+    46, 711, 764, 868, 53, 413, 278, 800, 255, 993, 318, 854, 319, 695, 315,
+    469, 166, 489, 969, 730, 1001, 757, 873, 686, 197, 303, 919, 155, 673,
+    940, 712, 25, 999, 63, 863, 972, 967, 785, 152, 296, 512, 402, 377, 45,
+    899, 829, 354, 77, 69, 856, 417, 811, 953, 124, 418, 75, 794, 162, 414,
+    1018, 568, 254, 265, 772, 588, 16, 896, 157, 889, 298, 621, 110, 844,
+    1000, 108, 545, 601, 78, 862, 447, 185, 195, 818, 450, 387, 49, 805, 102,
+    986, 1005, 827, 329, 28, 932, 410, 287, 435, 451, 962, 517, 48, 174, 43,
+    893, 884, 261, 251, 516, 395, 910, 611, 29, 501, 223, 476, 364, 144, 871,
+    998, 687, 928, 115, 453, 513, 176, 94, 168, 667, 955, 353, 434, 382, 400,
+    139, 365, 996, 343, 948, 890, 1012, 663, 610, 718, 538, 1008, 639, 470,
+    848, 543, 1011, 859, 671, 756, 83, 427, 159, 746, 669, 589, 971, 524,
+    356, 995, 904, 256, 201, 988, 62, 397, 81, 720, 917, 209, 549, 943, 486,
+    76, 148, 207, 509, 644, 386, 700, 534, 177, 550, 961, 926, 546, 428, 284,
+    127, 294, 8, 269, 359, 506, 445, 997, 806, 591, 725, 178, 262, 846, 373,
+    831, 504, 305, 843, 553, 378, 1017, 783, 474, 683, 581, 200, 498, 694,
+    191, 217, 847, 941, 424, 235, 38, 74, 616, 786, 147, 4, 273, 214, 142,
+    575, 992, 463, 983, 243, 360, 970, 350, 267, 615, 766, 494, 31, 1009,
+    452, 710, 552, 128, 612, 600, 275, 322, 193
+};
+static_assert(std::size(GF1024_LOG) == 1024, "GF1024_EXP length should be 1024");
+
 /* Determine the final constant to use for the specified encoding. */
 uint32_t EncodingConstant(Encoding encoding) {
     assert(encoding == Encoding::BECH32 || encoding == Encoding::BECH32M);
@@ -127,12 +276,73 @@ uint32_t PolyMod(const data& v)
     return c;
 }
 
+uint32_t Syndrome(const uint32_t residue) {
+    uint32_t low = residue & 0x1f;
+    return low ^ (low << 10) ^ (low << 20) ^
+        ((residue >> 5) & 1 ? 0x31edd3c4 : 0) ^
+        ((residue >> 6) & 1 ? 0x335f86a8 : 0) ^
+        ((residue >> 7) & 1 ? 0x363b8870 : 0) ^
+        ((residue >> 8) & 1 ? 0x3e6390c9 : 0) ^
+        ((residue >> 9) & 1 ? 0x2ec72192 : 0) ^
+        ((residue >> 10) & 1 ? 0x1046f79d : 0) ^
+        ((residue >> 11) & 1 ? 0x208d4e33 : 0) ^
+        ((residue >> 12) & 1 ? 0x130ebd6f : 0) ^
+        ((residue >> 13) & 1 ? 0x2499fade : 0) ^
+        ((residue >> 14) & 1 ? 0x1b27d4b5 : 0) ^
+        ((residue >> 15) & 1 ? 0x04be1eb4 : 0) ^
+        ((residue >> 16) & 1 ? 0x0968b861 : 0) ^
+        ((residue >> 17) & 1 ? 0x1055f0c2 : 0) ^
+        ((residue >> 18) & 1 ? 0x20ab4584 : 0) ^
+        ((residue >> 19) & 1 ? 0x1342af08 : 0) ^
+        ((residue >> 20) & 1 ? 0x24f1f318 : 0) ^
+        ((residue >> 21) & 1 ? 0x1be34739 : 0) ^
+        ((residue >> 22) & 1 ? 0x35562f7b : 0) ^
+        ((residue >> 23) & 1 ? 0x3a3c5bff : 0) ^
+        ((residue >> 24) & 1 ? 0x266c96f7 : 0) ^
+        ((residue >> 25) & 1 ? 0x25c78b65 : 0) ^
+        ((residue >> 26) & 1 ? 0x1b1f13ea : 0) ^
+        ((residue >> 27) & 1 ? 0x34baa2f4 : 0) ^
+        ((residue >> 28) & 1 ? 0x3b61c0e1 : 0) ^
+        ((residue >> 29) & 1 ? 0x265325c2 : 0);
+}
+
 /** Convert to lower case. */
 inline unsigned char LowerCase(unsigned char c)
 {
     return (c >= 'A' && c <= 'Z') ? (c - 'A') + 'a' : c;
 }
 
+void push_range(int from, int to, std::vector<int>& vec)
+{
+    for (int i = from; i < to; i++) {
+        vec.push_back(i);
+    }
+}
+
+/** Return index of first invalid character in a Bech32 string. */
+bool CheckCharacters(const std::string& str, std::vector<int>& errors) {
+    bool lower = false, upper = false;
+    for (size_t i = 0; i < str.size(); ++i) {
+        unsigned char c = str[i];
+        if (c >= 'a' && c <= 'z') {
+            if (upper) {
+                errors.push_back(i);
+            } else {
+                lower = true;
+            }
+        } else if (c >= 'A' && c <= 'Z') {
+            if (lower) {
+                errors.push_back(i);
+            } else {
+                upper = true;
+            }
+        } else if (c < 33 || c > 126) {
+            errors.push_back(i);
+        }
+    }
+    return errors.empty();
+}
+
 /** Expand a HRP for use in checksum computation. */
 data ExpandHRP(const std::string& hrp)
 {
@@ -196,14 +406,8 @@ std::string Encode(Encoding encoding, const std::string& hrp, const data& values
 
 /** Decode a Bech32 or Bech32m string. */
 DecodeResult Decode(const std::string& str) {
-    bool lower = false, upper = false;
-    for (size_t i = 0; i < str.size(); ++i) {
-        unsigned char c = str[i];
-        if (c >= 'a' && c <= 'z') lower = true;
-        else if (c >= 'A' && c <= 'Z') upper = true;
-        else if (c < 33 || c > 126) return {};
-    }
-    if (lower && upper) return {};
+    std::vector<int> errors;
+    if (!CheckCharacters(str, errors)) return {};
     size_t pos = str.rfind('1');
     if (str.size() > 90 || pos == str.npos || pos == 0 || pos + 7 > str.size()) {
         return {};
@@ -227,4 +431,103 @@ DecodeResult Decode(const std::string& str) {
     return {result, std::move(hrp), data(values.begin(), values.end() - 6)};
 }
 
+/** Find index of an incorrect character in a Bech32 string. */
+std::string LocateErrors(const std::string& str, std::vector<int>& error_locations) {
+    if (str.size() > 90) {
+        push_range(90, str.size(), error_locations);
+        return "Bech32 string too long";
+    }
+    if (!CheckCharacters(str, error_locations)){
+        return "Invalid character or mixed case";
+    }
+    size_t pos = str.rfind('1');
+    if (pos == str.npos) {
+        return "Missing separator";
+    }
+    if (pos == 0 || pos + 7 > str.size()) {
+        error_locations.push_back(pos);
+        return "Invalid separator position";
+    }
+    std::string hrp;
+    for (size_t i = 0; i < pos; ++i) {
+        hrp += LowerCase(str[i]);
+    }
+
+    size_t length = str.size() - 1 - pos; // length of data part
+    data values(length);
+    for (size_t i = pos + 1; i < str.size(); ++i) {
+        unsigned char c = str[i];
+        int8_t rev = CHARSET_REV[c];
+        if (rev == -1) {
+            error_locations.push_back(i);
+            return "Invalid Base 32 character";
+        }
+        values[i - pos - 1] = rev;
+    }
+
+    // We attempt error detection with both bech32 and bech32m, and choose the one with the fewest errors
+    // We can't simply use the segwit version, because that may be one of the errors
+    for (Encoding encoding : {Encoding::BECH32, Encoding::BECH32M}) {
+        std::vector<int> possible_errors;
+        uint32_t residue = PolyMod(Cat(ExpandHRP(hrp), values)) ^ EncodingConstant(encoding);
+        if (residue != 0) {
+            uint32_t syn = Syndrome(residue);
+            int s0 = syn & 0x3FF;
+            int s1 = (syn >> 10) & 0x3FF;
+            int s2 = syn >> 20;
+            int l_s0 = GF1024_LOG[s0];
+            int l_s1 = GF1024_LOG[s1];
+            int l_s2 = GF1024_LOG[s2];
+
+            if (l_s0 != -1 && l_s1 != -1 && l_s2 != -1 && (2 * l_s1 - l_s2 - l_s0 + 2046) % 1023 == 0) {
+                size_t p1 = (l_s1 - l_s0 + 1023) % 1023;
+                int l_e1 = l_s0 + (1023 - 997) * p1;
+                if (p1 < length && !(l_e1 % 33)) {
+                    possible_errors.push_back(str.size() - p1 - 1);
+                }
+            } else {
+                for (size_t p1 = 0; p1 < length; ++p1) {
+                    int s2_s1p1 = s2 ^ (s1 == 0 ? 0 : GF1024_EXP[(l_s1 + p1) % 1023]);
+                    if (s2_s1p1 == 0) continue;
+
+                    int s1_s0p1 = s1 ^ (s0 == 0 ? 0 : GF1024_EXP[(l_s0 + p1) % 1023]);
+                    if (s1_s0p1 == 0) continue;
+
+                    int l_s1_s0p1 = GF1024_LOG[s1_s0p1];
+                    size_t p2 = (GF1024_LOG[s2_s1p1] - l_s1_s0p1 + 1023) % 1023;
+                    if (p2 >= length || p1 == p2) continue;
+
+                    int s1_s0p2 = s1 ^ (s0 == 0 ? 0 : GF1024_EXP[(l_s0 + p2) % 1023]);
+                    if (s1_s0p2 == 0) continue;
+
+                    int inv_p1_p2 = 1023 - GF1024_LOG[GF1024_EXP[p1] ^ GF1024_EXP[p2]];
+                    int l_e2 = l_s1_s0p1 + inv_p1_p2 + (1023 - 997) * p2;
+                    if (l_e2 % 33) continue;
+
+                    int l_e1 = GF1024_LOG[s1_s0p2] + inv_p1_p2 + (1023 - 997) * p1;
+                    if (l_e1 % 33) continue;
+
+                    if (p1 > p2) {
+                        possible_errors.push_back(str.size() - p1 - 1);
+                        possible_errors.push_back(str.size() - p2 - 1);
+                    } else {
+                        possible_errors.push_back(str.size() - p2 - 1);
+                        possible_errors.push_back(str.size() - p1 - 1);
+                    }
+                    break;
+                }
+            }
+        } else {
+            // No errors
+            error_locations.clear();
+            return "";
+        }
+
+        if (error_locations.empty() || (!possible_errors.empty() && possible_errors.size() < error_locations.size())) {
+            error_locations = std::move(possible_errors);
+        }
+    }
+    return "Invalid checksum";
+}
+
 } // namespace bech32