Bug #21437: Date#hash may return different values for equal dates with large years

• Status: Closed
• Assignee: -
• Target version: -
• ruby -v: 3.2.8 - 3.4.4 (and older versions too)
• Backport: 3.2: WONTFIX, 3.3: DONE, 3.4: DONE

[Stranger6667 (Dmitry Dygalo)]

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.

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:

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);
}

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.

[Stranger6667 (Dmitry Dygalo)]

• Description updated (diff)

[byroot (Jean Boussier)]

• Backport changed from 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN to 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED

• Status changed from Open to Closed

Applied in changeset git|c1877d431e76f4a782d51602fa8487e98d302956.

---

[ruby/date] [Bug #21437] Date#hash for large years

Addresses https://bugs.ruby-lang.org/issues/21437

Signed-off-by: Dmitry Dygalo [e-mail]

https://github.com/ruby/date/commit/31f07bc576

[nagachika (Tomoyuki Chikanaga)]

• Backport changed from 3.2: WONTFIX, 3.3: REQUIRED, 3.4: REQUIRED to 3.2: WONTFIX, 3.3: DONE, 3.4: REQUIRED

ruby_3_3 1fb5ab411f618f651926ecae9427605e03e19884 merged revision(s) c1877d431e76f4a782d51602fa8487e98d302956.

[k0kubun (Takashi Kokubun)]

• Backport changed from 3.2: WONTFIX, 3.3: DONE, 3.4: REQUIRED to 3.2: WONTFIX, 3.3: DONE, 3.4: DONE

ruby_3_4 02744e58f3d410931a58873e1dab8752740e60ff merged revision(s) c1877d431e76f4a782d51602fa8487e98d302956.