From e1706f242d8512057ca420179452f8e45375d114 Mon Sep 17 00:00:00 2001
From: Alexey Andreev <konsoletyper@gmail.com>
Date: Thu, 14 Sep 2023 20:55:09 +0200
Subject: [PATCH] classlib: improve accuracy of Double.toString

---
 .../classlib/impl/text/DoubleAnalyzer.java    |  4 +-
 .../classlib/impl/text/DoubleSynthesizer.java | 51 +++++++++++++++++--
 2 files changed, 49 insertions(+), 6 deletions(-)

diff --git a/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleAnalyzer.java b/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleAnalyzer.java
index b13bed31b..3e52e0c71 100644
--- a/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleAnalyzer.java
+++ b/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleAnalyzer.java
@@ -47,7 +47,7 @@ public final class DoubleAnalyzer {
             mantissa |= 1L << 52;
         }
 
-        int decExponent = Arrays.binarySearch(exp10Table, exponent);
+        int decExponent = Arrays.binarySearch(exp10Table, (short) exponent);
         if (decExponent < 0) {
             decExponent = -decExponent;
         }
@@ -810,7 +810,7 @@ public final class DoubleAnalyzer {
             -1468012460592228864L,
     };
 
-    private static int[] exp10Table = {
+    static short[] exp10Table = {
             -70,
             -66,
             -63,
diff --git a/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleSynthesizer.java b/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleSynthesizer.java
index 5a2cc7b92..71f4dc43d 100644
--- a/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleSynthesizer.java
+++ b/classlib/src/main/java/org/teavm/classlib/impl/text/DoubleSynthesizer.java
@@ -26,7 +26,7 @@ public final class DoubleSynthesizer {
         }
 
         var binMantissa = DoubleAnalyzer.mulAndShiftRight(mantissa, mantissa10Table[indexInTable], 0);
-        var binExp = exp10Table[indexInTable];
+        int binExp = exp10Table[indexInTable];
 
         var binMantissaShift = (64 - Long.numberOfLeadingZeros(binMantissa)) - 58;
         if (binMantissaShift >= 0) {
@@ -39,7 +39,27 @@ public final class DoubleSynthesizer {
         if (binExp >= 2047) {
             return negative ? Double.NEGATIVE_INFINITY : Double.POSITIVE_INFINITY;
         }
-        binMantissa += 1L << 4;
+
+        var mantissaLowerBits = 5;
+        var mantissaLowerPos = 1 << mantissaLowerBits;
+        var error = (int) (binMantissa & (mantissaLowerPos - 1));
+        var correction = mantissaLowerPos >> 1;
+        if (Math.abs(error - (mantissaLowerPos >>> 1)) <= 1) {
+            var binMantissaWithoutError = binMantissa & -mantissaLowerPos;
+            var low = calcDecMantissa(binMantissaWithoutError, mantissaLowerPos, indexInTable, binExp);
+            var hi = calcDecMantissa(binMantissaWithoutError + mantissaLowerPos, mantissaLowerPos,
+                    indexInTable, binExp);
+            low = mantissa - low;
+            hi = hi - mantissa;
+            var cmp = Long.compareUnsigned(low, hi);
+            if (cmp < 0) {
+                correction = -error;
+            } else if (cmp > 0) {
+                correction = mantissaLowerPos - error;
+            }
+        }
+
+        binMantissa += correction;
         if ((binMantissa & (-1L << 58L)) != 0) {
             binMantissa >>>= 1;
             binExp++;
@@ -49,7 +69,7 @@ public final class DoubleSynthesizer {
             binExp = 0;
         }
 
-        binMantissa = (binMantissa >>> 5) & (-1L << 12 >>> 12);
+        binMantissa = (binMantissa >>> mantissaLowerBits) & (-1L << 12 >>> 12);
         var iee754 = binMantissa | ((long) binExp << 52);
         if (negative) {
             iee754 ^= 1L << 63;
@@ -57,6 +77,29 @@ public final class DoubleSynthesizer {
         return Double.longBitsToDouble(iee754);
     }
 
+    private static long calcDecMantissa(long mantissa, int lowerBit, int indexInTable, int binExp) {
+        var half = lowerBit >>> 1;
+        var shift = 7 - (DoubleAnalyzer.exp10Table[indexInTable] - binExp);
+        var decMantissa = DoubleAnalyzer.mulAndShiftRight(mantissa,
+                DoubleAnalyzer.mantissa10Table[indexInTable], shift);
+        var decMantissaHi = DoubleAnalyzer.mulAndShiftRight(mantissa + half,
+                DoubleAnalyzer.mantissa10Table[indexInTable], shift);
+        var decMantissaLow = DoubleAnalyzer.mulAndShiftRight(mantissa - half,
+                DoubleAnalyzer.mantissa10Table[indexInTable], shift);
+
+        var lowerPos = DoubleAnalyzer.findLowerDistance(decMantissa, decMantissaLow);
+        var upperPos = DoubleAnalyzer.findUpperDistance(decMantissa, decMantissaHi);
+        var posCmp = Long.compareUnsigned(lowerPos, upperPos);
+        if (posCmp > 0) {
+            decMantissa = Long.divideUnsigned(decMantissa, lowerPos) * lowerPos;
+        } else if (posCmp < 0) {
+            decMantissa = Long.divideUnsigned(decMantissa, upperPos) * upperPos + upperPos;
+        } else {
+            decMantissa = Long.divideUnsigned(decMantissa + (upperPos / 2), upperPos) * upperPos;
+        }
+        return decMantissa;
+    }
+
     // Numbers in the table below are generated by DoubleSynthesizerGenerator
 
     private static final long[] mantissa10Table = {
@@ -722,7 +765,7 @@ public final class DoubleSynthesizer {
             -8425902273664687726L,
     };
 
-    private static int[] exp10Table = {
+    private static short[] exp10Table = {
             -76,
             -72,
             -69,