Format: fixes substring with an end index outside of the string

`{1~20,30}` printed the placeholder as is, because the `,end` part was only
checked when the start index was inside the string. `{1~20}` printed nothing.
Parse and check the whole range first, so that a valid range that lies outside
of the string gives an empty result.

CHANGELOG.md

@@ -12,6 +12,7 @@ Bugfixes:
 * Fixed substring formatting returning the remaining string when a negative end index is before the start of the string. (General)
 * Fixed a skipped `{?}` / `{/}` conditional in a format string ending at the `{?}` / `{/}` of a conditional nested in it. (General)
 * Fixed numbers in format strings that don't fit in 32 bits wrapping around to a valid argument index, constant index, substring index or length, e.g. `{4294967297}` being read as `{1}`, and a signed integer overflow for `{1:-2147483648}`. (General)
+* Fixed substring formatting printing the placeholder as is, instead of nothing, when the start index is outside of the string and an end index is given. (General)
 
 Logos:
 * Added Tiny Core Linux (#2655)

src/common/impl/format.c

@@ -592,15 +592,22 @@ static bool parseFormatString(FFstrbuf* buffer, const FFstrbuf* formatstr, uint3
 
             char* pEnd = nullptr;
             int32_t start = strToInt32(pSep + 1, &pEnd);
-            if (start < 0) {
-                start = (int32_t) tempString.length + start;
+            bool hasEnd = *pEnd == ',';
+            int32_t end = 0;
+            if (hasEnd) {
+                end = strToInt32(pEnd + 1, &pEnd);
             }
-            if (start >= 0 && (uint32_t) start < tempString.length) {
-                if (*pEnd == '\0') {
-                    ffStrbufAppendNS(buffer, tempString.length - (uint32_t) start, &tempString.chars[start]);
-                } else if (*pEnd == ',') {
-                    int32_t end = strToInt32(pEnd + 1, &pEnd);
-                    if (!*pEnd) {
+
+            // Check the whole range before using it, so that a valid range that lies
+            // outside of the string gives an empty result instead of an invalid placeholder
+            if (!*pEnd) {
+                if (start < 0) {
+                    start = (int32_t) tempString.length + start;
+                }
+                if (start >= 0 && (uint32_t) start < tempString.length) {
+                    if (!hasEnd) {
+                        ffStrbufAppendNS(buffer, tempString.length - (uint32_t) start, &tempString.chars[start]);
+                    } else {
                         if (end < 0) {
                             end = (int32_t) tempString.length + end;
                         }

tests/format.c

@@ -113,6 +113,10 @@ int main(void) {
         VERIFY("output({1~-5,-3})", "12345 67890", "output(67)");
         VERIFY("output({1~-0,10})", "12345 67890", "output(12345 6789)");
         VERIFY("output({1~-3,-5})", "12345 67890", "output()");
+        VERIFY("output({1~11,12})", "12345 67890", "output()");  // Same as {1~11}
+        VERIFY("output({1~20,30})", "12345 67890", "output()");  // Same as {1~20}
+        VERIFY("output({1~-50,3})", "12345 67890", "output()");  // Same as {1~-50}
+        VERIFY("output({1~-50,-1})", "12345 67890", "output()"); // Same as {1~-50}
         VERIFY("output({1~})", "12345 67890", "output(12345 67890)");   // Same as {1~0}
         VERIFY("output({1~-0})", "12345 67890", "output(12345 67890)"); // Same as {1~0}
         VERIFY("output({1~,-1})", "12345 67890", "output(12345 6789)"); // Same as {1~0,-1}