[yugabyte#21830] DocDB: Import the usearch library

Summary:
Import the usearch header-only library and its dependency, the fp16 library. Both of these libraries are header-only.

Introducing an inline-thirdparty directory in the src directory where we can easily import header-only libraries by copying the relevant header sources. We can switch to git subtrees, or create a tool automate pulling in upstream changes into our own git repository later.
Jira: DB-10732

Test Plan:
Jenkins: test regex: usearch

New test: usearch_vector_index-test

Reviewers: tnayak

Reviewed By: tnayak

Subscribers: jason, ybase

Differential Revision: https://phorge.dev.yugabyte.com/D33682

CMakeLists.txt

@@ -409,7 +409,6 @@ if(IS_GCC)
     # https://gist.githubusercontent.com/mbautin/de18543ea85d46db49dfa4b4b7df082a/raw
     ADD_CXX_FLAGS("-Wno-use-after-free")
   endif()
-
 endif()
 
 if(USING_LINUXBREW)
@@ -600,10 +599,16 @@ file(MAKE_DIRECTORY "${CMAKE_LIBRARY_OUTPUT_DIRECTORY}")
 set(EXECUTABLE_OUTPUT_PATH "${YB_BUILD_ROOT}/bin")
 file(MAKE_DIRECTORY "${EXECUTABLE_OUTPUT_PATH}")
 
-# Generated sources always have higher priority.
+# Generated sources always have higher priority than identically named sources in the source
+# directory.
 include_directories(${CMAKE_CURRENT_BINARY_DIR}/src)
+
 include_directories(src)
 
+include_directories("src/inline-thirdparty/usearch")
+include_directories("src/inline-thirdparty/fp16")
+
+
 enable_testing()
 
 if (USING_LINUXBREW)

src/inline-thirdparty/README.md

@@ -0,0 +1,20 @@
+# inline-thirdparty
+
+This is a directory where we copy some of the third-party header-only libraries, rather than adding
+them to the yugabyte-db-thirdparty repo. We also only copy the relevant subdirectory of the upstream
+repositories. Each library is copied in its own appropriately named directory, and each library's
+directory is added separately to the list of include directories in CMakeLists.txt.
+
+* usearch
+  * Repo: https://github.com/yugabyte/usearch
+  * Description: Similarity search for vector and text
+  * Subdirectory: include
+  * Tag: v2.11.0-yb-1
+  * License: Apache 2.0
+
+* fp16
+  * Repo: https://github.com/Maratyszcza/FP16/
+  * Description: Header-only library for conversion to/from half-precision floating point formats
+  * Subdirectory: include
+  * Commit: 0a92994d729ff76a58f692d3028ca1b64b145d91
+  * License: MIT

src/inline-thirdparty/fp16/fp16.h

@@ -0,0 +1,11 @@
+#pragma once
+#ifndef FP16_H
+#define FP16_H
+
+#include <fp16/fp16.h>
+
+#if defined(PSIMD_H)
+#include <fp16/psimd.h>
+#endif
+
+#endif /* FP16_H */

src/inline-thirdparty/fp16/fp16/bitcasts.h

@@ -0,0 +1,92 @@
+#pragma once
+#ifndef FP16_BITCASTS_H
+#define FP16_BITCASTS_H
+
+#if defined(__cplusplus) && (__cplusplus >= 201103L)
+	#include <cstdint>
+#elif !defined(__OPENCL_VERSION__)
+	#include <stdint.h>
+#endif
+
+#if defined(__INTEL_COMPILER)
+	#include <immintrin.h>
+#endif
+
+#if defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
+	#include <intrin.h>
+#endif
+
+
+static inline float fp32_from_bits(uint32_t w) {
+#if defined(__OPENCL_VERSION__)
+	return as_float(w);
+#elif defined(__CUDA_ARCH__)
+	return __uint_as_float((unsigned int) w);
+#elif defined(__INTEL_COMPILER)
+	return _castu32_f32(w);
+#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
+	return _CopyFloatFromInt32((__int32) w);
+#else
+	union {
+		uint32_t as_bits;
+		float as_value;
+	} fp32 = { w };
+	return fp32.as_value;
+#endif
+}
+
+static inline uint32_t fp32_to_bits(float f) {
+#if defined(__OPENCL_VERSION__)
+	return as_uint(f);
+#elif defined(__CUDA_ARCH__)
+	return (uint32_t) __float_as_uint(f);
+#elif defined(__INTEL_COMPILER)
+	return _castf32_u32(f);
+#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
+	return (uint32_t) _CopyInt32FromFloat(f);
+#else
+	union {
+		float as_value;
+		uint32_t as_bits;
+	} fp32 = { f };
+	return fp32.as_bits;
+#endif
+}
+
+static inline double fp64_from_bits(uint64_t w) {
+#if defined(__OPENCL_VERSION__)
+	return as_double(w);
+#elif defined(__CUDA_ARCH__)
+	return __longlong_as_double((long long) w);
+#elif defined(__INTEL_COMPILER)
+	return _castu64_f64(w);
+#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
+	return _CopyDoubleFromInt64((__int64) w);
+#else
+	union {
+		uint64_t as_bits;
+		double as_value;
+	} fp64 = { w };
+	return fp64.as_value;
+#endif
+}
+
+static inline uint64_t fp64_to_bits(double f) {
+#if defined(__OPENCL_VERSION__)
+	return as_ulong(f);
+#elif defined(__CUDA_ARCH__)
+	return (uint64_t) __double_as_longlong(f);
+#elif defined(__INTEL_COMPILER)
+	return _castf64_u64(f);
+#elif defined(_MSC_VER) && (defined(_M_ARM) || defined(_M_ARM64))
+	return (uint64_t) _CopyInt64FromDouble(f);
+#else
+	union {
+		double as_value;
+		uint64_t as_bits;
+	} fp64 = { f };
+	return fp64.as_bits;
+#endif
+}
+
+#endif /* FP16_BITCASTS_H */