From 6b2f3ad6c83f255f00e73b8ec9b21aab5d4edfbd Mon Sep 17 00:00:00 2001
From: Tim Ledbetter <timledbetter@gmail.com>
Date: Fri, 24 Feb 2023 20:29:14 +0000
Subject: [PATCH] AK: Fix DeprecatedString::bijective_base_from for large
 numbers

The output of the DeprecatedString::bijective_base_from() is now
correct for numbers larger than base^2.

This makes column names display correctly in Spreadsheet.
---
 AK/DeprecatedString.cpp           | 18 ++++++++++--------
 Tests/AK/TestDeprecatedString.cpp |  5 ++++-
 2 files changed, 14 insertions(+), 9 deletions(-)

diff --git a/AK/DeprecatedString.cpp b/AK/DeprecatedString.cpp
index 5877bc74265..437b27a71ab 100644
--- a/AK/DeprecatedString.cpp
+++ b/AK/DeprecatedString.cpp
@@ -246,6 +246,7 @@ DeprecatedString DeprecatedString::repeated(StringView string, size_t count)
 
 DeprecatedString DeprecatedString::bijective_base_from(size_t value, unsigned base, StringView map)
 {
+    value++;
     if (map.is_null())
         map = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"sv;
 
@@ -255,15 +256,16 @@ DeprecatedString DeprecatedString::bijective_base_from(size_t value, unsigned ba
     Array<char, round_up_to_power_of_two(sizeof(size_t) * 8 + 1, 2)> buffer;
     size_t i = 0;
     do {
-        buffer[i++] = map[value % base];
-        value /= base;
-    } while (value > 0);
+        auto remainder = value % base;
+        auto new_value = value / base;
+        if (remainder == 0) {
+            new_value--;
+            remainder = map.length();
+        }
 
-    // NOTE: Weird as this may seem, the thing that comes after 'Z' is 'AA', which as a number would be '00'
-    //       to make this work, only the most significant digit has to be in a range of (1..25) as opposed to (0..25),
-    //       but only if it's not the only digit in the string.
-    if (i > 1)
-        --buffer[i - 1];
+        buffer[i++] = map[remainder - 1];
+        value = new_value;
+    } while (value > 0);
 
     for (size_t j = 0; j < i / 2; ++j)
         swap(buffer[j], buffer[i - j - 1]);
diff --git a/Tests/AK/TestDeprecatedString.cpp b/Tests/AK/TestDeprecatedString.cpp
index a0045357e3e..0b6687f4064 100644
--- a/Tests/AK/TestDeprecatedString.cpp
+++ b/Tests/AK/TestDeprecatedString.cpp
@@ -290,7 +290,10 @@ TEST_CASE(bijective_base)
     EXPECT_EQ(DeprecatedString::bijective_base_from(25), "Z");
     EXPECT_EQ(DeprecatedString::bijective_base_from(26), "AA");
     EXPECT_EQ(DeprecatedString::bijective_base_from(52), "BA");
-    EXPECT_EQ(DeprecatedString::bijective_base_from(704), "ABC");
+    EXPECT_EQ(DeprecatedString::bijective_base_from(701), "ZZ");
+    EXPECT_EQ(DeprecatedString::bijective_base_from(702), "AAA");
+    EXPECT_EQ(DeprecatedString::bijective_base_from(730), "ABC");
+    EXPECT_EQ(DeprecatedString::bijective_base_from(18277), "ZZZ");
 }
 
 TEST_CASE(roman_numerals)