Emit shortest round-trip decimal for numbers (RFC 8785 §3.2.2.3)

lib/json/canonicalization.rb

@@ -33,9 +33,29 @@ def to_json_c14n
     if num < 0
       num, sign = -num, '-'
     end
-    native_rep = "%.15E" % num
-    decimal, exponential = native_rep.split('E')
-    exp_val = exponential.to_i
+    # Decompose into the shortest decimal digit sequence that round-trips,
+    # per RFC 8785 §3.2.2.3 (ECMAScript Number-to-String). Float#to_s already
+    # yields that shortest sequence; the previous "%.15E" gave only 16
+    # significant digits and could not round-trip doubles that need 17
+    # (e.g. 0.1 + 0.2). `decimal` is normalised to a single leading digit and
+    # `exp_val` is the power of ten of that digit, matching the shape the
+    # formatting below expects.
+    rep = num.to_s
+    if rep =~ /e/i
+      mantissa, power = rep.split(/e/i)
+      int_part, frac_part = mantissa.split('.')
+      digits = int_part + frac_part.to_s
+      exp_val = power.to_i
+    else
+      int_part, frac_part = rep.split('.')
+      all_digits = int_part + frac_part.to_s
+      first = (all_digits =~ /[1-9]/) || all_digits.length - 1
+      exp_val = int_part.length - 1 - first
+      digits = all_digits[first..]
+    end
+    digits = digits.sub(/0+\z/, '')
+    digits = '0' if digits.empty?
+    decimal = digits[0] + '.' + (digits.length > 1 ? digits[1..] : '0')
     exponential = exp_val > 0 ? ('+' + exp_val.to_s) : exp_val.to_s
 
     integral, fractional = decimal.split('.')

spec/number_spec.rb

@@ -17,13 +17,20 @@
     999999999999999700000     => '999999999999999700000',
     999999999999999900000     => '999999999999999900000',
     333333333.33333329        => '333333333.3333333',
-    # -5e-324                 => '-5e-324', # Outside Ruby Range
-    # 1.0000000000000001e+23  => '1.0000000000000001e+23', # Outside Ruby Range
-    # 295147905179352830000   => '295147905179352830000', # Outside Ruby Range
-    #-1.7976931348623157e+308 => '-1.7976931348623157e+308',  # Outside Ruby Range
-    #1.7976931348623157e+308  => '1.7976931348623157e+308',  # Outside Ruby Range
-    #1e+23                    => '1e+23', # Outside Ruby
-    #5e-324                   => '5e-324',  # Outside Ruby Range
+    # These are representable doubles that need 17 significant digits; they
+    # were previously mis-serialised (only 16 digits were emitted) and so had
+    # been left commented out. They now round-trip correctly.
+    -5e-324                   => '-5e-324',
+    1.0000000000000001e+23    => '1.0000000000000001e+23',
+    295147905179352830000     => '295147905179352830000',
+    -1.7976931348623157e+308  => '-1.7976931348623157e+308',
+    1.7976931348623157e+308   => '1.7976931348623157e+308',
+    1e+23                     => '1e+23',
+    5e-324                    => '5e-324',
+    # Round-trip regressions: the canonical form must parse back to the same
+    # double. "%.15E" produced '0.3' for 0.1 + 0.2, a different value.
+    0.1 + 0.2                 => '0.30000000000000004',
+    123456789012345680.0      => '123456789012345680',
   }.each do |data, expected|
     if expected.is_a?(String)
       it "converts #{data} to #{expected}" do