Add Jenkins hash to supported hash algorithms. (#2233)

Add the option to configure the hash algorithm to the lower or upper 16
bits of a Jenkins hash algorithm
(https://en.wikipedia.org/wiki/Jenkins_hash_function). This provides a
stronger alternative to the CRC-based algorithms where supported.

Signed-off-by: Mike Beresford <mberes@google.com>

Author: mikeberesford

doc/Hash/Jenkins_hash_configuration.md

@@ -0,0 +1,127 @@
+# [SAI] Jenkins Hash Algorithm
+
+--------------------------------------------------------------------------------
+
+Title       | Jenkins Hash Algorithm
+----------- | ---------------------------------------
+Authors     | Mike Beresford, Abhishek Verma (Google)
+Status      | In-Review
+Type        | Standards track
+Created     | 2026-02-05
+SAI-Version | 1.18
+
+--------------------------------------------------------------------------------
+
+## 1. Introduction
+
+ECMP or Next hop groups (NHG) and LAG use load balancing techniques to spread
+the traffic across the various members. The most common load balancing technique
+is to use hash-based member selection. In this technique, flows are identified
+by performing a hash on a set of packet fields like the 5-tuple, and then
+selecting the group member based on the computed hash value.
+
+This proposal introduces the addition of Jenkins hash algorithm to
+_sai_hash_algorithm_t.
+
+## 2. Motivation
+
+Limitation of CRC/XOR: Traditional CRC and XOR algorithms are susceptible to
+polarization when traffic passes through multiple next hops.
+
+Jenkins hash function is a non-cryptographic hash function that offers better
+avalanche properties compared to CRC and XOR. This results in a more uniform
+distribution of flows across members, especially in multi-stage networks (e.g.,
+CLOS topologies) where polarization is a concern.
+
+## 3. Behavior
+
+The Jenkins algorithm (specifically `lookup3`) produces a 32-bit hash value for
+a given input (e.g., packet 5-tuple). Switching ASICs generally rely on a 16-bit
+hash value for load balancing. To accommodate this, the 32-bit Jenkins hash is
+split into two 16-bit segments.
+
+Two new enums are introduced to allow selecting either the lower 16 bits
+(`_LO`) or the upper 16 bits (`_HI`) of the computed hash. This provides
+flexibility, for example allowing different hash values for ECMP and LAG even
+if the same flow keys are used.
+
+## 4. SAI Enhancement
+
+New enums added to the `sai_hash_algorithm_t`.
+
+Enum defining the Jenkins hash algorithm:
+
+```c
+/**
+ * @brief Attribute data for #SAI_SWITCH_ATTR_ECMP_DEFAULT_HASH_ALGORITHM
+ * and #SAI_SWITCH_ATTR_LAG_DEFAULT_HASH_ALGORITHM
+ */
+typedef enum _sai_hash_algorithm_t
+{
+    ...
+
+    /** Round-robin based hash algorithm (per-packet round-robin spraying) */
+    SAI_HASH_ALGORITHM_ROUND_ROBIN = 8,
+
+    /** Lower 16 bits of Jenkins hash algorithm */
+    SAI_HASH_ALGORITHM_JENKINS_LO = 9,
+
+    /** Upper 16 bits of Jenkins hash algorithm */
+    SAI_HASH_ALGORITHM_JENKINS_HI = 10,
+
+} sai_hash_algorithm_t;
+
+```
+
+## 5. API Example(s)
+
+### Query supported hashing algorithms
+
+```c
+    sai_s32_list_t enum_values_capability;
+    enum_values_capability.count = 0;
+    enum_values_capability.list = nullptr;
+
+   // Fetch the number of supported hash algorithms, then allocate a buffer and 
+   // fetch the actual list.
+    sai_status_t status = sai->queryAttributeEnumValuesCapability(
+        switchid,
+        SAI_OBJECT_TYPE_SWITCH,
+        SAI_SWITCH_ATTR_ECMP_DEFAULT_HASH_ALGORITHM,
+        &enum_values_capability);
+
+    std::vector<int32_t> list;
+    list.resize(enum_values_capability.count);
+    enum_values_capability.list = list.data();
+    sai_status_t status = sai->queryAttributeEnumValuesCapability(
+        switchid,
+        SAI_OBJECT_TYPE_SWITCH,
+        SAI_SWITCH_ATTR_ECMP_DEFAULT_HASH_ALGORITHM,
+        &enum_values_capability);
+
+    ASSERT_EQ(status, SAI_STATUS_SUCCESS);
+
+    bool jenkins_hash_lo_supported = false;
+    for (uint32_t i = 0; i < enum_values_capability.count; ++i)
+    {
+        if (enum_values_capability.list[i] == SAI_HASH_ALGORITHM_JENKINS_LO)
+        {
+            jenkins_hash_lo_supported = true;
+        }
+    }
+```
+
+### Set Jenkins algorithm as the default hash algorithm
+
+```c
+    sai_attribute_t attr;
+    attr.id = SAI_SWITCH_ATTR_ECMP_DEFAULT_HASH_ALGORITHM;
+    attr.value.s32 = SAI_HASH_ALGORITHM_JENKINS_LO;
+    status = sai->set(SAI_OBJECT_TYPE_SWITCH, switchid, &attr);
+    ASSERT_EQ(status, SAI_STATUS_SUCCESS);
+```
+
+## 6. References
+
+https://burtleburtle.net/bob/c/lookup3.c
+

inc/saiswitch.h

@@ -214,6 +214,12 @@ typedef enum _sai_hash_algorithm_t
     /** Round-robin based hash algorithm (per-packet round-robin spraying) */
     SAI_HASH_ALGORITHM_ROUND_ROBIN = 8,
 
+    /** Lower 16 bits of Jenkins hash algorithm */
+    SAI_HASH_ALGORITHM_JENKINS_LO = 9,
+
+    /** Upper 16 bits of Jenkins hash algorithm */
+    SAI_HASH_ALGORITHM_JENKINS_HI = 10,
+
 } sai_hash_algorithm_t;
 
 /**