LibJS: Update ToRawPrecision / ToRawFixed AO spec comments

This is a normative change in the Intl spec:
https://github.com/tc39/ecma402/commit/f0f66cf

There are two main changes here:
1. Converting BigInt/Number objects to mathematical values.
2. A change in how ToRawPrecision computes its exponent and significant
   digits.

For (1), we do not yet support BigInt number formatting, thus already
have coerced Number objects to a double. When BigInt is supported, the
number passed into these methods will likely still be a Value, thus can
be coereced then.

For (2), our implementation already returns the expected edge-case
results pointed out on the spec PR.

1 files changed, 28 insertions, 23 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp b/Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp
index c8524d1b4c..4561b0d933 100644
--- a/Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp
@@ -1129,11 +1129,14 @@ RawFormatResult to_raw_precision(double number, int min_precision, int max_preci
 {
     RawFormatResult result {};
 
-    // 1. Let p be maxPrecision.
+    // 1. Set x to ℝ(x).
+    // FIXME: Support BigInt number formatting.
+
+    // 2. Let p be maxPrecision.
     int precision = max_precision;
     int exponent = 0;
 
-    // 2. If x = 0, then
+    // 3. If x = 0, then
     if (number == 0.0) {
         // a. Let m be the String consisting of p occurrences of the character "0".
         result.formatted_string = String::repeated('0', precision);
@@ -1144,31 +1147,28 @@ RawFormatResult to_raw_precision(double number, int min_precision, int max_preci
         // c. Let xFinal be 0.
         result.rounded_number = 0;
     }
-
-    // 3. Else,
+    // 4. Else,
     else {
         // FIXME: The result of these steps isn't entirely accurate for large values of 'p' (which
         //        defaults to 21, resulting in numbers on the order of 10^21). Either AK::format or
         //        our Number::toString AO (double_to_string in Value.cpp) will need to be improved
         //        to produce more accurate results.
 
-        // a. Let e be the base 10 logarithm of x rounded down to the nearest integer.
+        // a. Let e and n be integers such that 10^(p–1) ≤ n < 10^p and for which n × 10^(e–p+1) – x is as close to zero as possible.
+        //    If there are two such sets of e and n, pick the e and n for which n × 10^(e–p+1) is larger.
         exponent = log10floor(number);
 
         double power = pow(10, exponent - precision + 1);
-
-        // b. Let n be an integer such that 10^(p–1) ≤ n < 10^p and for which the exact mathematical value of n × 10^(e–p+1) – x
-        //    is as close to zero as possible. If there is more than one such n, pick the one for which n × 10^(e–p+1) is larger.
         double n = round(number / power);
 
-        // c. Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
+        // b. Let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
         result.formatted_string = Value(n).to_string_without_side_effects();
 
-        // d. Let xFinal be n × 10^(e–p+1).
+        // c. Let xFinal be n × 10^(e–p+1).
         result.rounded_number = n * power;
     }
 
-    // 4. If e ≥ p–1, then
+    // 5. If e ≥ p–1, then
     if (exponent >= (precision - 1)) {
         // a. Let m be the string-concatenation of m and e–p+1 occurrences of the character "0".
         result.formatted_string = String::formatted(
@@ -1179,7 +1179,7 @@ RawFormatResult to_raw_precision(double number, int min_precision, int max_preci
         // b. Let int be e+1.
         result.digits = exponent + 1;
     }
-    // 5. Else if e ≥ 0, then
+    // 6. Else if e ≥ 0, then
     else if (exponent >= 0) {
         // a. Let m be the string-concatenation of the first e+1 characters of m, the character ".", and the remaining p–(e+1) characters of m.
         result.formatted_string = String::formatted(
@@ -1190,7 +1190,7 @@ RawFormatResult to_raw_precision(double number, int min_precision, int max_preci
         // b. Let int be e+1.
         result.digits = exponent + 1;
     }
-    // 6. Else,
+    // 7. Else,
     else {
         // a. Assert: e < 0.
         // b. Let m be the string-concatenation of the String value "0.", –(e+1) occurrences of the character "0", and m.
@@ -1203,15 +1203,16 @@ RawFormatResult to_raw_precision(double number, int min_precision, int max_preci
         result.digits = 1;
     }
 
-    // 7. If m contains the character ".", and maxPrecision > minPrecision, then
+    // 8. If m contains the character ".", and maxPrecision > minPrecision, then
     if (result.formatted_string.contains('.') && (max_precision > min_precision)) {
         // a. Let cut be maxPrecision – minPrecision.
         int cut = max_precision - min_precision;
 
+        // Steps 8b-8c are implemented by cut_trailing_zeroes.
         result.formatted_string = cut_trailing_zeroes(result.formatted_string, cut);
     }
 
-    // 8. Return the Record { [[FormattedString]]: m, [[RoundedNumber]]: xFinal, [[IntegerDigitsCount]]: int }.
+    // 9. Return the Record { [[FormattedString]]: m, [[RoundedNumber]]: xFinal, [[IntegerDigitsCount]]: int }.
     return result;
 }
 
@@ -1221,21 +1222,24 @@ RawFormatResult to_raw_fixed(double number, int min_fraction, int max_fraction)
 {
     RawFormatResult result {};
 
-    // 1. Let f be maxFraction.
+    // 1. Set x to ℝ(x).
+    // FIXME: Support BigInt number formatting.
+
+    // 2. Let f be maxFraction.
     int fraction = max_fraction;
 
     double power = pow(10, fraction);
 
-    // 2. Let n be an integer for which the exact mathematical value of n / 10^f – x is as close to zero as possible. If there are two such n, pick the larger n.
+    // 3. Let n be an integer for which the exact mathematical value of n / 10^f – x is as close to zero as possible. If there are two such n, pick the larger n.
     double n = round(number * power);
 
-    // 3. Let xFinal be n / 10^f.
+    // 4. Let xFinal be n / 10^f.
     result.rounded_number = n / power;
 
-    // 4. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
+    // 5. If n = 0, let m be the String "0". Otherwise, let m be the String consisting of the digits of the decimal representation of n (in order, with no leading zeroes).
     result.formatted_string = n == 0.0 ? String("0"sv) : Value(n).to_string_without_side_effects();
 
-    // 5. If f ≠ 0, then
+    // 6. If f ≠ 0, then
     if (fraction != 0) {
         // a. Let k be the number of characters in m.
         auto decimals = result.formatted_string.length();
@@ -1262,17 +1266,18 @@ RawFormatResult to_raw_fixed(double number, int min_fraction, int max_fraction)
         // e. Let int be the number of characters in a.
         result.digits = a.length();
     }
-    // 6. Else, let int be the number of characters in m.
+    // 7. Else, let int be the number of characters in m.
     else {
         result.digits = result.formatted_string.length();
     }
 
-    // 7. Let cut be maxFraction – minFraction.
+    // 8. Let cut be maxFraction – minFraction.
     int cut = max_fraction - min_fraction;
 
+    // Steps 9-10 are implemented by cut_trailing_zeroes.
     result.formatted_string = cut_trailing_zeroes(result.formatted_string, cut);
 
-    // 10. Return the Record { [[FormattedString]]: m, [[RoundedNumber]]: xFinal, [[IntegerDigitsCount]]: int }.
+    // 11. Return the Record { [[FormattedString]]: m, [[RoundedNumber]]: xFinal, [[IntegerDigitsCount]]: int }.
     return result;
 }