[jsinterp] Add `js_number_to_string` (#12110)

Authored by: Grub4K

1 files changed, 55 insertions, 0 deletions

diff --git a/yt_dlp/jsinterp.py b/yt_dlp/jsinterp.py
index ba059babb..bd6a47004 100644
--- a/yt_dlp/jsinterp.py
+++ b/yt_dlp/jsinterp.py
@@ -95,6 +95,61 @@ def _js_ternary(cndn, if_true=True, if_false=False):
     return if_true
 
 
+# Ref: https://es5.github.io/#x9.8.1
+def js_number_to_string(val: float, radix: int = 10):
+    if radix in (JS_Undefined, None):
+        radix = 10
+    assert radix in range(2, 37), 'radix must be an integer at least 2 and no greater than 36'
+
+    if math.isnan(val):
+        return 'NaN'
+    if val == 0:
+        return '0'
+    if math.isinf(val):
+        return '-Infinity' if val < 0 else 'Infinity'
+    if radix == 10:
+        # TODO: implement special cases
+        ...
+
+    ALPHABET = b'0123456789abcdefghijklmnopqrstuvwxyz.-'
+
+    result = collections.deque()
+    sign = val < 0
+    val = abs(val)
+    fraction, integer = math.modf(val)
+    delta = max(math.nextafter(.0, math.inf), math.ulp(val) / 2)
+
+    if fraction >= delta:
+        result.append(-2)  # `.`
+    while fraction >= delta:
+        delta *= radix
+        fraction, digit = math.modf(fraction * radix)
+        result.append(int(digit))
+        # if we need to round, propagate potential carry through fractional part
+        needs_rounding = fraction > 0.5 or (fraction == 0.5 and int(digit) & 1)
+        if needs_rounding and fraction + delta > 1:
+            for index in reversed(range(1, len(result))):
+                if result[index] + 1 < radix:
+                    result[index] += 1
+                    break
+                result.pop()
+
+            else:
+                integer += 1
+            break
+
+    integer, digit = divmod(int(integer), radix)
+    result.appendleft(digit)
+    while integer > 0:
+        integer, digit = divmod(integer, radix)
+        result.appendleft(digit)
+
+    if sign:
+        result.appendleft(-1)  # `-`
+
+    return bytes(ALPHABET[digit] for digit in result).decode('ascii')
+
+
 # Ref: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Operator_Precedence
 _OPERATORS = {  # None => Defined in JSInterpreter._operator
     '?': None,