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ch2-6: 完善 qsort 例子
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chai2010
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* [2.3. 类型转换](ch2-cgo/ch2-03-cgo-types.md)
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* [2.4. 函数调用](ch2-cgo/ch2-04-func.md)
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* [2.5. 内部机制](ch2-cgo/ch2-05-internal.md)
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* [2.6. 实战: 封装qsort(TODO)](ch2-cgo/ch2-06-qsort.md)
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* [2.6. 实战: 封装qsort(Doing)](ch2-cgo/ch2-06-qsort.md)
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* [2.7. CGO内存模型](ch2-cgo/ch2-07-memory.md)
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* [2.8. C++类包装](ch2-cgo/ch2-08-class.md)
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* [2.9. 静态库和动态库](ch2-cgo/ch2-09-static-shared-lib.md)
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@ -1,3 +1,259 @@
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# 2.6. 实战: 封装qsort
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qsort快速排序函数是C语言的高阶函数，支持用于自定义排序比较函数，可以对任意类型的数组进行排序。本节我们尝试基于C语言的qsort函数封装一个Go语言版本的qsort函数。
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## 认识qsort函数
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qosrt快速排序函数有`<stdlib.h>`标准库提供，函数的声明如下：
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```c
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void qsort(
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void* base, size_t num, size_t size,
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int (*cmp)(const void*, const void*)
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);
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```
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其中base参数是要排序数组的首个元素的地址，num是数组中元素的个数，size是数组中每个元素的大小。最关键是cmp比较函数，用于对数组中任意两个元素进行排序。cmp排序函数的两个指针参数分别是要比较的两个元素的地址，如果第一个参数对应元素大于第二个参数对应的元素将返回1，如果两个元素相等则返回0，如果第一个元素小于第二个元素则返回-1。
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下面的例子是用C语言的qsort对一个int类型的数组进行排序：
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```c
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#include <stdio.h>
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#include <stdlib.h>
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#define DIM(x) (sizeof(x)/sizeof((x)[0]))
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static int cmp(const void* a, const void* b) {
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const int* pa = (int*)a;
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const int* pb = (int*)b;
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return *pa - *pb;
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}
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int main() {
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int values[] = { 42, 8, 109, 97, 23, 25 };
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int i;
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qsort(values, DIM(values), sizeof(values[0]), cmp);
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for(i = 0; i < DIM(values); i++) {
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printf ("%d ",values[i]);
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}
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return 0;
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}
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```
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## 用于Go固定类型数组的排序
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```go
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/*
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#include <stdlib.h>
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#define DIM(x) (sizeof(x)/sizeof((x)[0]))
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static int compare(const void* a, const void* b) {
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return ( *(int*)a - *(int*)b );
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}
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static void qsort_proxy(int values[], size_t len, size_t elemsize) {
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qsort(values, DIM(values), sizeof(values[0]), compare);
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}
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*/
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import "C"
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import "unsafe"
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import "fmt"
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func main() {
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values := []int32{ 42, 9, 101, 95, 27, 25 };
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C.qsort_proxy(
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unsafe.Pointer(&values[0]),
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C.size_t(len(values)),
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C.size_t(unsafe.Sizeof(values[0])),
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)
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fmt.Println(values)
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}
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```
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## 在Go中自传入比较函数
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```go
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/*
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extern int go_qsort_compare(void* a, void* b);
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static int compare(const void* a, const void* b) {
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return go_qsort_compare((void*)(a), (void*)(b))
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}
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*/
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import "C"
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//export go_qsort_compare
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func go_qsort_compare(a, b unsafe.Pointer) C.int {
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pa := (*C.int)(a)
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pb := (*C.int)(b)
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return C.int(*pa - *pb)
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}
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```
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```go
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/*
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#include <stdlib.h>
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typedef int (*qsort_cmp_func_t)(const void* a, const void* b);
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extern int go_qsort_compare(void* a, void* b);
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*/
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import "C"
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func main() {
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values := []int32{42, 9, 101, 95, 27, 25}
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C.qsort(unsafe.Pointer(&values[0]),
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C.size_t(len(values)), C.size_t(unsafe.Sizeof(values[0])),
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(C.qsort_cmp_func_t)(unsafe.Pointer(C.go_qsort_compare)),
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)
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}
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```
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```go
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import "C"
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//export go_qsort_compare
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func go_qsort_compare(a, b unsafe.Pointer) C.int {
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return go_qsort_compare_info.fn(a, b)
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}
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var go_qsort_compare_info struct {
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fn func(a, b unsafe.Pointer) C.int
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sync.RWMutex
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}
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```
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```go
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func main() {
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values := []int32{42, 9, 101, 95, 27, 25}
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go_qsort_compare_info.Lock()
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defer go_qsort_compare_info.Unlock()
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go_qsort_compare_info.fn = func(a, b unsafe.Pointer) C.int {
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pa := (*C.int)(a)
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pb := (*C.int)(b)
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return C.int(*pa - *pb)
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}
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C.qsort(unsafe.Pointer(&values[0]),
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C.size_t(len(values)), C.size_t(unsafe.Sizeof(values[0])),
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(C.qsort_cmp_func_t)(unsafe.Pointer(C.go_qsort_compare)),
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)
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}
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```
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```go
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func qsort(values []int32, fn func(a, b unsafe.Pointer) C.int) {
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go_qsort_compare_info.Lock()
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defer go_qsort_compare_info.Unlock()
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go_qsort_compare_info.fn = fn
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C.qsort(
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unsafe.Pointer(&values[0]),
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C.size_t(len(values)),
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C.size_t(unsafe.Sizeof(values[0])),
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(C.qsort_cmp_func_t)(unsafe.Pointer(C.go_qsort_compare)),
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)
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}
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```
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## Go比较函数类型的简化
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## 适配更多数组类型
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```go
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func qsort(slice interface{}, fn func(a, b unsafe.Pointer) C.int) {
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sv := reflect.ValueOf(slice)
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if sv.Kind() != reflect.Slice {
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panic("not slice type")
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}
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go_qsort_compare_info.Lock()
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defer go_qsort_compare_info.Unlock()
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go_qsort_compare_info.fn = fn
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C.qsort(
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unsafe.Pointer(unsafe.Pointer(sv.Index(0).Addr().Pointer())),
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C.size_t(sv.Len()), C.size_t(sv.Type().Elem().Size()),
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(C.qsort_cmp_func_t)(unsafe.Pointer(C.go_qsort_compare)),
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)
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}
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```
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```go
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func qsort(slice interface{}, fn func(a, b int) int) {
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...
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}
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```
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```go
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/*
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#include <stdlib.h>
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extern int go_qsort_compare(void* a, void* b);
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extern void go_qsort_compare_save_base(void* base);
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static void qsort_proxy(
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void* base, size_t num, size_t size,
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int (*compar)(const void* a, const void* b)
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) {
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go_qsort_compare_save_base(base); // 保存数组地址
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qsort(base, num, size, compar);
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}
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*/
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import "C"
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```
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```go
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//export go_qsort_compare_save_base
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func go_qsort_compare_save_base(base unsafe.Pointer) {
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go_qsort_compare_info.base = uintptr(base)
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}
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var go_qsort_compare_info struct {
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base uintptr
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elemsize uintptr
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fn func(a, b int) int
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sync.RWMutex
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}
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```
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```go
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//export go_qsort_compare
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func go_qsort_compare(a, b unsafe.Pointer) C.int {
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var (
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// array memory is locked
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base = go_qsort_compare_info.base
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elemsize = go_qsort_compare_info.elemsize
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)
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i := int((uintptr(a) - base) / elemsize)
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j := int((uintptr(b) - base) / elemsize)
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return C.int(go_qsort_compare_info.fn(i, j))
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}
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```
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```go
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func main() {
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values := []int64{42, 9, 101, 95, 27, 25}
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qsort(values, func(i, j int) int {
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return int(values[i] - values[j])
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})
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}
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func qsort(slice interface{}, fn func(a, b int) int) {
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...
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}
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```
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