From: "Stranger6667 (Dmitry Dygalo) via ruby-core" <ruby-core@...>
Date: 2025-06-11T12:50:20+00:00
Subject: [ruby-core:122516] [Ruby Bug#21437] Date#hash may return different values for equal dates with large years

Issue #21437 has been reported by Stranger6667 (Dmitry Dygalo).

----------------------------------------
Bug #21437: Date#hash may return different values for equal dates with large years
https://bugs.ruby-lang.org/issues/21437

* Author: Stranger6667 (Dmitry Dygalo)
* Status: Open
* ruby -v: 3.2.8 - 3.4.4 (and older versions too)
* Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Parsing the same date string allocates new large integer objects with different pointer addresses, which then affects the hash value due to `d_lite_hash` behavior.

```ruby
require 'date'

# Small year - consistent hashes
Date.strptime('2024-5-9', '%Y-%m-%d').hash
#=> 3287407234087466140
Date.strptime('2024-5-9', '%Y-%m-%d').hash  
#=> 3287407234087466140

# BUG - Large year - different hashes for identical date strings
Date.strptime('3171505571716611468830131104691-5-9', '%Y-%m-%d').hash
#=> 3525034678868094350
Date.strptime('3171505571716611468830131104691-5-9', '%Y-%m-%d').hash
#=> 2502076152602341316  # Different hash for same date!
```

In [ext/date/date_core.c](https://github.com/ruby/ruby/blob/v3_4_4/ext/date/date_core.c#L6936):

```c
static VALUE
d_lite_hash(VALUE self)
{
    st_index_t v, h[4];
    get_d1(self);
    h[0] = m_nth(dat);  // Could be different for equal large integers
    // ...
    v = rb_memhash(h, sizeof(h));
    return ST2FIX(v);
}
```

Each call to `Date.strptime` with a large year allocates a new large integer for `m_nth(dat)`. The hash function uses this pointer address directly in `rb_memhash`, so equal dates parsed separately get different hashes.

This breaks the hash contract that equal objects must have equal hashes.



-- 
https://bugs.ruby-lang.org/
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