From 323c9b4df6df00f5242df39432e85ba1d95a349a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Sim=C3=B5es?= <jose.simoes@eclo.solutions>
Date: Fri, 31 Oct 2025 03:08:50 +0000
Subject: [PATCH 1/3] Reworked rounding logic in nanoprintf

- Reworked the rounding logic to use the actual rounding digit (pend) and propagate carry correctly from the last significant digit leftwards.
- Properly handles ripple (e.g., 9.8999 -> 10) and updates decpt/ndigits when needed in FF_FCVT mode.
---
 src/CLR/Helpers/nanoprintf/nanoprintf.c | 51 ++++++++++++++-----------
 1 file changed, 28 insertions(+), 23 deletions(-)

diff --git a/src/CLR/Helpers/nanoprintf/nanoprintf.c b/src/CLR/Helpers/nanoprintf/nanoprintf.c
index a25294c366..bbad167e85 100644
--- a/src/CLR/Helpers/nanoprintf/nanoprintf.c
+++ b/src/CLR/Helpers/nanoprintf/nanoprintf.c
@@ -418,34 +418,39 @@ static char *format_float(double number, flt_width_t ndigits, flt_width_t width,
     {
         p -= (i - FLOAT_DIGITS);
     }
-    if (!(fflags & FF_NRND) && *p >= '5')
+    // Use the rounding digit at 'pend' (the extra generated digit)
+    if (!(fflags & FF_NRND) && *pend >= '5')
     {
-        for (;;)
+        char *start = buf +2; // first generated digit position
+        char *q = pend -1; // last significant digit
+
+        // Set rounding digit to zero; it will be dropped later
+        *pend = '0';
+
+        // Propagate carry leftwards
+        while (q >= start && *q == '9')
         {
-            if (i == 0)
-            {
-                // The rounding has rippled all the way through to
-                // the first digit. i.e. 9.999..9 -> 10.0
-                // Just replace the first 0 with a 1 and shift the DP.
-                *p = '1';
-                ++decpt;
-                // This increases the displayed digits for 'f' only.
-                if ((fflags & (FF_FCVT|FF_GCVT)) == FF_FCVT)
-                {
-                    ++ndigits;
-                    ++pend;
-                }
-                break;
-            }
-            // Previous digit was a rollover
-            *p-- = '0';
-            // Increment next digit and break out unless there is a rollover.
-            if (*p != '9')
+            *q = '0';
+            --q;
+        }
+
+        if (q < start)
+        {
+            // Rollover past the most significant digit, e.g.,9.999 ->10.000
+            *start = '1';
+            ++decpt;
+            // In fixed 'f' mode, this increases displayed digits
+            if ((fflags & (FF_FCVT | FF_GCVT)) == FF_FCVT)
             {
-                (*p)++;
-                break;
+                ++ndigits;
+                ++pend; // account for the added most significant digit
             }
         }
+        else
+        {
+            // Normal increment without rollover
+            (*q)++;
+        }
     }
 
     // Insert the decimal point

From 7d0cd4e8258abd892589d2f4a804281e52a08bcf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Sim=C3=B5es?= <jose.simoes@eclo.solutions>
Date: Fri, 31 Oct 2025 04:16:56 +0000
Subject: [PATCH 2/3] Handle negative 0 situation

---
 src/CLR/CorLib/corlib_native_System_Number.cpp | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/src/CLR/CorLib/corlib_native_System_Number.cpp b/src/CLR/CorLib/corlib_native_System_Number.cpp
index 34a9939a03..b47b20ec3b 100644
--- a/src/CLR/CorLib/corlib_native_System_Number.cpp
+++ b/src/CLR/CorLib/corlib_native_System_Number.cpp
@@ -718,10 +718,11 @@ int Library_corlib_native_System_Number::Format_F(
 
     ret = DoPrintfOnDataType(buffer, formatStr, value);
 
+    bool isNegative = (buffer[0] == '-');
+
     // this extra processing is only required for integer types
     if (isIntegerDataType && ret > 0)
     {
-        bool isNegative = (buffer[0] == '-');
         int offsetBecauseOfNegativeSign = (isNegative ? 1 : 0);
 
         int dotIndex = GetDotIndex(buffer, ret);
@@ -756,6 +757,13 @@ int Library_corlib_native_System_Number::Format_F(
         ret = ReplaceNegativeSign(buffer, ret, negativeSign);
         ret = ReplaceDecimalSeparator(buffer, ret, decimalSeparator);
     }
+    else if (isNegative && ret == 2 && buffer[1] == '0')
+    {
+        // handle negative zero: if the value was negative but rounds to 0, remove the minus sign
+        // remove the negative sign
+        memmove(buffer, &buffer[1], ret);
+        ret--;
+    }
 
     return ret;
 }

From 20f32a6bbbf4713ccf49e31fb13fc945a7b49d56 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jos=C3=A9=20Sim=C3=B5es?= <jose.simoes@eclo.solutions>
Date: Fri, 31 Oct 2025 04:30:27 +0000
Subject: [PATCH 3/3] Handle edge case with precision and value as 0

---
 src/CLR/Helpers/nanoprintf/nanoprintf.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/src/CLR/Helpers/nanoprintf/nanoprintf.c b/src/CLR/Helpers/nanoprintf/nanoprintf.c
index bbad167e85..7dcb438153 100644
--- a/src/CLR/Helpers/nanoprintf/nanoprintf.c
+++ b/src/CLR/Helpers/nanoprintf/nanoprintf.c
@@ -915,6 +915,14 @@ static printf_t doprnt(void *context, void (*func)(char c, void *context), size_
                 }
 #endif
 #if FEATURE(USE_PRECISION)
+                // If precision explicitly set to 0 and value is 0, emit a single '0'
+                if (precision == 0 && uvalue == 0)
+                {
+                    *--p = '0';
+#if FEATURE(USE_ZERO_PAD)
+                    --fwidth;
+#endif
+                }
                 while (uvalue || precision > 0)
 #else
                 if (uvalue == 0)