fix: Fix multipleOf to work with decimals

[torkeln]

A more simplistic approach to validating floats, where we multiply both value and multipleOf with 10 until multipleOf does not have any fractions, and then compare them as integers. This should work since both numbers are fetched from a text string that can't be repeating. Could possibly help solve #397

[OrangeTux]

Pinging @Stranger6667.

Is there any way we can bring this issue forward? I'm affected as well by the bug #397. Let me know if I can help.

[OrangeTux]

I can confirm the fix works.

[OrangeTux]

I've to correct myself. The solution doesn't always work.

For example, the fix fails to validate that 78.0 is a multiple of 0.1.

If found this issue by fuzzing with cargo fuzz.
Here is a diff to replicate the bug:

diff --git a/jsonschema/src/keywords/multiple_of.rs b/jsonschema/src/keywords/multiple_of.rs
index 64c1509..3cba1df 100644
--- a/jsonschema/src/keywords/multiple_of.rs
+++ b/jsonschema/src/keywords/multiple_of.rs
@@ -100,6 +100,8 @@ mod tests {
     #[test_case(&json!({"multipleOf": 0.1}), &json!(1.2))]
     #[test_case(&json!({"multipleOf": 0.1}), &json!(1.3))]
     #[test_case(&json!({"multipleOf": 0.02}), &json!(1.02))]
+    #[test_case(&json!({"multipleOf": 0.1}), &json!(78.0))]
     fn multiple_of_is_valid(schema: &Value, instance: &Value) {
         tests_util::is_valid(schema, instance)
     }

[OrangeTux]

The core of this PR is this code:

while tmp_item.fract() != 0. {
    tmp_item *= 10.0;
    tmp_multiple_of *= 10.0;
}
tmp_item % tmp_multiple_of == 0.0

It returns false when tmp_item is 78.0 and tmp_multiple_of is 0.1.

Since tmp_item.fract() == 0, the execution jumps to the last line.
Interestingly, 78.0 % 0.1 returns 0.09999884. Whereas I would expect it returns 0.
See https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=61f390594bccbfee2e853495a30fd52b

[OrangeTux]

I found a fix. But I'm not sure yet if the fix works for all inputs.
The fix divides tmp_item by tmp_multiple_of and validates that the fractional part of the result equals to 0.0.

diff --git a/jsonschema/src/keywords/multiple_of.rs b/jsonschema/src/keywords/multiple_of.rs
index 64c1509..eb49b59 100644
--- a/jsonschema/src/keywords/multiple_of.rs
+++ b/jsonschema/src/keywords/multiple_of.rs
@@ -35,7 +35,9 @@ impl Validate for MultipleOfValidator {
                 tmp_multiple_of *= 10.0;
             }

-            tmp_item % tmp_multiple_of == 0.0
+           (tmp_item / tmp_multiple_of).fract() == 0.0
+
+
         } else {
             true
         }

[OrangeTux]

My solution doesn't work for very large numbers and a value for "mutlipleOf" that's below 1.
Consider the values 1e308 and 0.1. 1e308 / 0.1 == f32::inf, and therefore returns false.

diff --git a/jsonschema/src/keywords/multiple_of.rs b/jsonschema/src/keywords/multiple_of.rs
index 64c1509..3d431a8 100644
--- a/jsonschema/src/keywords/multiple_of.rs
+++ b/jsonschema/src/keywords/multiple_of.rs
@@ -100,6 +100,8 @@ mod tests {
     #[test_case(&json!({"multipleOf": 0.1}), &json!(1.2))]
     #[test_case(&json!({"multipleOf": 0.1}), &json!(1.3))]
     #[test_case(&json!({"multipleOf": 0.02}), &json!(1.02))]
+    #[test_case(&json!({"multipleOf": 0.5}), &json!(1e308))]
     fn multiple_of_is_valid(schema: &Value, instance: &Value) {
         tests_util::is_valid(schema, instance)
     }

[OrangeTux]

Here's a relevant discussion explaining why 78.0 % 0.1 is not 0.0: https://users.rust-lang.org/t/how-does-the-operator-work-with-floating-point-numbers/82030