From 6090a3a1b9a5379f960c87759b0c0f60b79ff503 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juan=20Manuel=20Tom=C3=A1s?= <jtomas1815@gmail.com>
Date: Thu, 26 Mar 2020 05:36:19 -0300
Subject: Implement page structure

---
 page.c | 89 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100644 page.c

diff --git a/page.c b/page.c
new file mode 100644
index 0000000..5d15f1d
--- /dev/null
+++ b/page.c
@@ -0,0 +1,89 @@
+#include <string.h>
+
+#define PAGE_SIZE 1024
+
+struct page {
+	char contents[PAGE_SIZE];
+	char *gap_start;
+	char *gap_end;
+	struct page *next;
+	struct page *prev;
+};
+
+struct page *new_page() {
+	struct page *result = malloc(sizeof(struct page));
+	result->gap_start = result->contents;
+	result->gap_end = result->contents + PAGE_SIZE - 1;
+	result->next = 0;
+	result->prev = 0;
+	return result;
+}
+
+static inline void set_half_page_gap(struct page *page) {
+	page->gap_start = page->contents + PAGE_SIZE / 2;
+	page->gap_end = page->contents + PAGE_SIZE - 1;
+}
+
+void split_page(struct page *first_half) {
+	struct page *second_half = new_page();
+
+	memcpy(second_half->contents, first_half->contents + PAGE_SIZE / 2, PAGE_SIZE / 2 - 1);
+
+	set_half_page_gap(first_half);
+	set_half_page_gap(second_half);
+
+	if (first_half->next) {
+		first_half->next->prev = second_half;
+	}
+	second_half->next = first_half->next;
+	second_half->prev = first_half;
+	first_half->next = second_half;
+}
+
+void delete_page(struct page *page) {
+	if (page->next) {
+		page->next->prev = page->prev;
+	}
+	if (page->prev) {
+		page->prev->next = page->next;
+	}
+	free(page);
+}
+
+void move_gap(struct page *page, int target) {
+	while(target) {
+		if (target > 0) {
+			page->gap_end++;
+			*(page->gap_start) = *(page->gap_end);
+			page->gap_start++;
+			target--;
+		} else {
+			page->gap_start--;
+			*(page->gap_end) = *(page->gap_start);
+			page->gap_end--;
+			target++;
+		}
+	}
+}
+
+static inline int page_is_full(struct page *page) {
+	return page->gap_start == page->gap_end;
+}
+
+static inline int page_is_empty(struct page *page) {
+	return page->gap_start == page->contents
+		&& page->gap_end   == page->contents + PAGE_SIZE - 1;
+}
+
+void insert_into_page(struct page *page, char c) {
+	if (!page_is_full(page)) {
+		*(page->gap_start) = c;
+		page->gap_start++;
+	}
+}
+
+void erase_from_page(struct page *page) {
+	if (page->gap_start != page->contents) {
+		page->gap_start--;
+	}
+}
-- 
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