[stdlib] Add a new validate parameter to the b64decode() function

docs/changelog.md

@@ -100,6 +100,8 @@ what we publish.
 
 ### Standard library changes
 
+- Add a new `validate` parameter to the `b64decode()` function.
+
 - The free floating functions for constructing different types have been
   deprecated for actual constructors:
 

stdlib/src/base64/base64.mojo

@@ -34,7 +34,7 @@ from ._b64encode import b64encode_with_buffers as _b64encode_with_buffers
 
 
 @always_inline
-fn _ascii_to_value(char: StringSlice) -> Int:
+fn _ascii_to_value[validate: Bool = False](char: StringSlice) raises -> Int:
     """Converts an ASCII character to its integer value for base64 decoding.
 
     Args:
@@ -58,6 +58,12 @@ fn _ascii_to_value(char: StringSlice) -> Int:
     elif char == "/":
         return 63
     else:
+
+        @parameter
+        if validate:
+            raise Error(
+                'ValueError: Unexpected character "{}" encountered'.format(char)
+            )
         return -1
 
 
@@ -112,31 +118,35 @@ fn b64encode(input_bytes: Span[Byte, _]) -> String:
 
 
 @always_inline
-fn b64decode(str: StringSlice) -> String:
+fn b64decode[validate: Bool = False](str: StringSlice) raises -> String:
     """Performs base64 decoding on the input string.
 
+    Parameters:
+      validate: If true, the function will validate the input string.
+
     Args:
       str: A base64 encoded string.
 
     Returns:
       The decoded string.
     """
     var n = str.byte_length()
-    debug_assert(n % 4 == 0, "Input length must be divisible by 4")
+
+    @parameter
+    if validate:
+        if n % 4 != 0:
+            raise Error(
+                "ValueError: Input length {} must be divisible by 4".format(n)
+            )
 
     var p = String._buffer_type(capacity=n + 1)
 
     # This algorithm is based on https://arxiv.org/abs/1704.00605
     for i in range(0, n, 4):
-        var a = _ascii_to_value(str[i])
-        var b = _ascii_to_value(str[i + 1])
-        var c = _ascii_to_value(str[i + 2])
-        var d = _ascii_to_value(str[i + 3])
-
-        debug_assert(
-            a >= 0 and b >= 0 and c >= 0 and d >= 0,
-            "Unexpected character encountered",
-        )
+        var a = _ascii_to_value[validate](str[i])
+        var b = _ascii_to_value[validate](str[i + 1])
+        var c = _ascii_to_value[validate](str[i + 2])
+        var d = _ascii_to_value[validate](str[i + 3])
 
         p.append((a << 2) | (b >> 4))
         if str[i + 2] == "=":

stdlib/test/base64/test_base64.mojo

@@ -14,7 +14,7 @@
 
 from base64 import b16decode, b16encode, b64decode, b64encode
 
-from testing import assert_equal
+from testing import assert_equal, assert_raises
 
 
 def test_b64encode():
@@ -60,6 +60,16 @@ def test_b64decode():
 
     assert_equal(b64decode("QUJDREVGYWJjZGVm"), "ABCDEFabcdef")
 
+    with assert_raises(
+        contains="ValueError: Input length 21 must be divisible by 4"
+    ):
+        _ = b64decode[validate=True]("invalid base64 string")
+
+    with assert_raises(
+        contains='ValueError: Unexpected character " " encountered'
+    ):
+        _ = b64decode[validate=True]("invalid base64 string!!!")
+
 
 def test_b16encode():
     assert_equal(b16encode("a"), "61")