feat(stack): introduce a stack ds, internally this uses a sll

README.md

@@ -45,6 +45,20 @@ ds_hmp_free(&hmp, NULL);
 free(b);
 ```
 
+## Stack
+```c
+char *a = "a";
+char *b = "b";
+
+ds_stack_t *stack = ds_stack_init();
+ds_stack_push(stack, a); // [ "a" ]
+ds_stack_push(stack, b); // [ "b", "a" ]
+ds_stack_peek(stack, 0); // "b"
+ds_stack_pop(stack); // "b"
+ds_stack_pop(stack); // "a"
+free(stack);
+```
+
 # TODO
 - [ ] more data structures
 - [x] tests

ds.c

@@ -367,3 +367,56 @@ void
 
   return NULL;
 }
+
+ds_stack_t
+*ds_stack_init(void)
+{
+  ds_stack_t *stack = malloc(sizeof(ds_stack_t));
+  stack->n = 0;
+
+  return stack;
+}
+
+int
+ds_stack_push(ds_stack_t *stack, void *data)
+{
+  if (stack->n == 0) {
+    stack->items = ds_sll_new_node(data);
+    stack->n++;
+    return 0;
+  }
+  int r = ds_sll_add(&stack->items, data, 0);
+  if (!r) {
+    stack->n++;
+  }
+  return r;
+}
+
+void
+*ds_stack_pop(ds_stack_t *stack)
+{
+  void *r = ds_sll_remove(&stack->items, 0);
+  if (r != NULL) {
+    stack->n--;
+  }
+  return r;
+}
+
+void
+*ds_stack_peek(ds_stack_t *stack, unsigned x)
+{
+  int i;
+  ds_sll_t *cur;
+
+  if (x > stack->n) {
+    return NULL;
+  }
+
+  for (cur = stack->items, i = 0; cur; cur = cur->next, i++) {
+    if (i == x) {
+      return cur->data;
+    }
+  }
+
+  return NULL;
+}

ds.h

@@ -22,6 +22,11 @@ typedef struct {
 	void *val;
 } _ds_hmp_kv_t;
 
+typedef struct {
+  unsigned  n;
+  ds_sll_t *items;
+} ds_stack_t;
+
 #define ds_ll_foreach(t, ll) for (t *cur = ll; cur; cur = cur->next)
 
 /**
@@ -168,4 +173,37 @@ void *ds_hmp_get(ds_hmp_t *hmp, char *key);
  */
 void *ds_hmp_remove(ds_hmp_t *hmp, char *key);
 
+/**
+ * @brief initialize a new stack
+ *
+ * @return the stack
+ */
+ds_stack_t *ds_stack_init(void);
+
+/**
+ * @brief push new data onto the stack
+ *
+ * @param stack the stack
+ * @param data the data
+ * @return 0 on success
+ */
+int ds_stack_push(ds_stack_t *stack, void *data);
+
+/**
+ * @brief pop the data off the top of the stack
+ *
+ * @param stack the stack
+ * @return the data
+ */
+void *ds_stack_pop(ds_stack_t *stack);
+
+/**
+ * @brief peek forward x items
+ *
+ * @param stack the stack
+ * @param x how many items to look ahead
+ * @return the data or NULL if out of range
+ */
+void *ds_stack_peek(ds_stack_t *stack, unsigned x);
+
 #endif

test.c

@@ -1,3 +1,4 @@
+#include <limits.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -76,4 +77,25 @@ main(int argc, char *argv[])
 
     free(a);
   );
+
+  test("stack",
+    char *a = "a";
+    char *b = "b";
+    char *c = "c";
+
+    ds_stack_t *stack = ds_stack_init();
+    it("pushes an item", ds_stack_push(stack, a) == 0);
+    it("pops an item", ds_stack_pop(stack) == a);
+
+    ds_stack_push(stack, b);
+    ds_stack_push(stack, c);
+
+    it("peeks ahead", ds_stack_peek(stack, 0) == c);
+    it("peeks past the stack", ds_stack_peek(stack, INT_MAX) == NULL);
+
+    ds_stack_pop(stack);
+    ds_stack_pop(stack);
+
+    free(stack);
+  );
 }