BUILD

cc_plugin(
    name = 'lda',
    srcs = ['lda.c'],
    deps = ['#pthread']
)

lda.c

#include "lda.h"

#include <stdlib.h>
#include <stdint.h>

OrderedSparseHistogram newHist(int dim) {
  OrderedSparseHistogram h;
  h.topics.array = malloc(sizeof(int32_t) * dim);
  h.topics.len = dim;
  h.topics.cap = dim;
  h.counts.array = malloc(sizeof(int64_t) * dim);
  h.counts.len = dim;
  h.counts.cap = dim;
  return h;
}

OrderedSparseHistogram Sample(Document* d) {
  uint32_t i = 0;
  Word* words = (Word*)(d->words.array);

  for (i = 0; i < d->words.len; ++i) {
    words[i].topic = 111;
  }

  OrderedSparseHistogram h = newHist(10);
  int32_t* topics = (int32_t*)(h.topics.array);
  int64_t* counts = (int64_t*)(h.counts.array);

  for (i = 0; i < h.topics.len; ++i) {
    topics[i] = i;
    counts[i] = i*10;
  }

  return h;
}

void FreeHist(OrderedSparseHistogram h) {
  free(h.topics.array);
  free(h.counts.array);
}

lda.go

package main

/*
 #cgo darwin CFLAGS: -I.
 #cgo darwin LDFLAGS: -L../../build64_release/src/learn-cgo/c -llda
 #include "c/lda.h"
 */
import "C"

import (
  "fmt"
	"unsafe"
)

type Word struct {
	word  int32
	topic int32
}

type OrderedSparseHistogram struct {
	topics    []int32
	counts []int64
}

type Document struct {
	words []Word
	hist  OrderedSparseHistogram
}

func CopyCHist(h C.OrderedSparseHistogram) OrderedSparseHistogram {
	r := OrderedSparseHistogram {
	topics: make([]int32, h.topics.len),
	counts: make([]int64, h.topics.len)}
	copy(r.topics, *(*[]int32)(unsafe.Pointer(&h.topics)))
	copy(r.counts, *(*[]int64)(unsafe.Pointer(&h.counts)))
	return r
}

func main() {
	d := Document{
	words: []Word{ {word:0, topic:0}, {word:1, topic:1}, {word:2, topic:2} },
	hist: OrderedSparseHistogram{
		topics: []int32{1,2,3},
		counts: []int64{9,9,9}}}

	fmt.Println(d)
	h := C.Sample((*C.Document)(unsafe.Pointer(&d)))
	d.hist = CopyCHist(h)
	C.FreeHist(h)
	fmt.Println(d)
}

lda.h

#ifndef LEARN_CGO_C_LDA_H_
#define LEARN_CGO_C_LDA_H_

#include <stdint.h>

typedef struct {
  int32_t word;
  int32_t topic;
} Word;

typedef struct {
  void* array;
  uint32_t len;
  uint32_t cap;
} Slice;

typedef struct {
  Slice topics;
  Slice counts;
} OrderedSparseHistogram;

typedef struct {
  Slice words;
  OrderedSparseHistogram hist;
} Document;

OrderedSparseHistogram Sample(Document*);

void FreeHist(OrderedSparseHistogram);

#endif  // LEARN_CGO_C_LDA_H_