Fix GEOHASH last character: always output '0' for Redis compatibility by Copilot · Pull Request #1635 · microsoft/garnet

Copilot · 2026-03-17T19:11:30Z

Garnet's GEOHASH produces a wrong 11th character because geohash strings require 55 bits (11 × 5) but geo scores are stored as 52-bit integers — the final 3 bits don't exist. Redis sets the last character to '0' by convention; Garnet was computing it from undefined bit positions.

Changes

GeoHash.cs — GetGeoHashCode: force index 0 on the last iteration instead of shifting undefined bits:

var idx = i < chars.Length - 1
    ? (int)(hash >> (BitsOfPrecision - 5)) & 0x1F
    : 0;
chars[i] = (char)base32Chars[idx];

GeoHashTests.cs: update all expected geohash strings — last character is now always '0' (e.g. wm3vxz6vywh → wm3vxz6vyw0, zzzzzzzzzzs → zzzzzzzzzz0); update comment to clarify the '0' convention.
RespSortedSetGeoTests.cs: update expected Palermo geohash sqc8b49rnys → sqc8b49rny0.

Original prompt

This section details on the original issue you should resolve

<issue_title>GEOHASH returns incorrect last character due to missing 52-bit truncation handling</issue_title>
<issue_description>### GEOHASH returns incorrect last character due to missing 52-bit truncation handling

Problem

Garnet's GEOHASH command produces a different final character than Redis for certain coordinates. For example:
GEOADD mygeo 13.361389 38.115556 "Palermo"
GEOHASH mygeo Palermo
Server Result

Redis sqc8b49rny0

Garnet sqc8b49rnys

The first 10 characters match, but the 11th diverges.

Root Cause

A standard geohash string is 11 base32 characters, which requires 55 bits (11 × 5). Both Redis and Garnet store geo data as sorted se
t scores using 64-bit doubles, which only have 52 bits of significand — 3 bits short of what the final character needs.

Redis accounts for this in src/geo.c lines 921–925 by setti
ng the 11th character to '0', since there aren't enough stored bits to determine it:
if (i == 10) {
    /* We have just 52 bits, but the API used to output
     * an 11 bytes geohash. For compatibility we assume
     * zero. */
    idx = 0;
}
Garnet's GetGeoHashCode
doesn't have this special case — it shifts the remaining 2 bits into the high bits of a 5-bit index, so the last character ends up re
flecting bit positions that don't carry real precision:
for (var i = 0; i < chars.Length; i++)
{
    chars[i] = (char)base32Chars[(int)(hash >> (BitsOfPrecision - 5)) & 0x1F];
    hash <<= 5;
}
Why It Matters

Clients and applications that compare, cache, or index geohash strings will see mismatches against values produced by Redis or other
standard implementations. Since the 52-bit score can't fully determine the 11th character, outputting '0' — as Redis does — is the
established convention.

Suggested Fix

On the last iteration of GetGeoHashCode, set the index to 0 rather than reading the remaining bits:
for (var i = 0; i < chars.Length; i++)
{
    var idx = i < chars.Length - 1
        ? (int)(hash >> (BitsOfPrecision - 5)) & 0x1F
        : 0;
    chars[i] = (char)base32Chars[idx];
    hash <<= 5;
}
```</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@badrishc</author><body>
@PaulusParssinen - what do you think?</body></comment_new>
</comments>

Fixes GEOHASH returns incorrect last character due to missing 52-bit truncation handling #1625

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Co-authored-by: badrishc <18355833+badrishc@users.noreply.github.com>

badrishc · 2026-03-17T20:33:58Z

Request your review on this PR, @sspeaks and @PaulusParssinen

Copilot

Pull request overview

Updates Garnet’s GEOHASH output to match Redis behavior by forcing the 11th (final) geohash character to '0', since Garnet stores only 52 bits of precision (insufficient for the full 55 bits implied by 11 base32 characters).

Changes:

Adjust GeoHash.GetGeoHashCode to force the last base32 character index to 0 (so the last character is always '0').
Update geohash expectations in unit/integration tests to reflect the new Redis-compatible output.
Change the pinned .NET SDK version in global.json from 10.0.103 to 10.0.102.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
libs/server/Objects/SortedSetGeo/GeoHash.cs	Forces the last geohash character to `'0'` for Redis compatibility under 52-bit precision.
test/Garnet.test/GeoHashTests.cs	Updates expected geohash strings and clarifies the `'0'` last-character convention.
test/Garnet.test/RespSortedSetGeoTests.cs	Updates expected GEOHASH output in RESP sorted set geo tests (e.g., Palermo).
global.json	Updates the repo’s pinned .NET SDK patch version.

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global.json

  "sdk": {
-    "version": "10.0.103",
+    "version": "10.0.102",
    "rollForward": "latestMajor",


PaulusParssinen · 2026-03-17T21:01:29Z

libs/server/Objects/SortedSetGeo/GeoHash.cs

+                    // For compatibility with Redis, the last character is always '0' since the
+                    // remaining bits are not stored with sufficient precision to determine it.
+                    var idx = i < chars.Length - 1
+                        ? (int)(hash >> (BitsOfPrecision - 5)) & 0x1F


Consider instead something along the lines of

var base32Chars = "0123456789bcdefghjkmnpqrstuvwxyz"u8; // We have just 52 bits, but the API outputs an 11-character geohash (55 bits). // For compatibility with Redis, the last character is always '0'. chars[^1] = '0'; for (var i = 0; i < chars.Length - 1; i++) { // Shift and mask the five most significant bits for index to the base-32 table. chars[i] = (char)base32Chars[(int)(hash >> (BitsOfPrecision - 5)) & 0x1F]; // Shift the encoded bits out. hash <<= 5; }

PaulusParssinen

Added comment with alternative approach without branches in the inner loop, either will do the job.

All other GEO* functionality will still use the available 52-bit precision that is persisted in the sorted set.

Initial plan

1545037

Copilot AI assigned Copilot and badrishc Mar 17, 2026

Copilot started work on behalf of badrishc March 17, 2026 19:11 View session

Fix GEOHASH last character to always return '0' for Redis compatibility

4c55847

Co-authored-by: badrishc <18355833+badrishc@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix GEOHASH incorrect last character due to truncation handling~~ Fix GEOHASH last character: always output '0' for Redis compatibility Mar 17, 2026

Copilot finished work on behalf of badrishc March 17, 2026 19:23

Copilot AI requested a review from badrishc March 17, 2026 19:23

badrishc marked this pull request as ready for review March 17, 2026 20:32

Copilot AI review requested due to automatic review settings March 17, 2026 20:32

Copilot started reviewing on behalf of badrishc March 17, 2026 20:34 View session

Copilot AI reviewed Mar 17, 2026

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global.json

Comment on lines 2 to 4

"sdk": {

"version": "10.0.103",

"version": "10.0.102",

"rollForward": "latestMajor",

PaulusParssinen reviewed Mar 17, 2026

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PaulusParssinen approved these changes Mar 17, 2026

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Fix GEOHASH last character: always output '0' for Redis compatibility#1635

Fix GEOHASH last character: always output '0' for Redis compatibility#1635
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Copilot AI commented Mar 17, 2026 •

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