working except for overflow at end of doc

input.txt

@@ -1 +1,4 @@
 Hello, Zed!
+Foo, Bar!
+Test, test
+

main.c

@@ -2,46 +2,139 @@
 #include <stdbool.h>
 #include <string.h>
 #include <errno.h>
+#include <assert.h>
 
 #include <termios.h>
 
 #include <unistd.h>
 
-#define ZED_CAPACITY 1024
+#define EDITOR_CAPACITY 1024
 
 typedef struct {
-    char data[ZED_CAPACITY];
-    size_t count;
+    size_t begin;
+    size_t end;
+} Line;
+
+typedef struct {
+    char data[EDITOR_CAPACITY];
+    size_t data_count;
+    Line lines[EDITOR_CAPACITY + 10];
+    size_t lines_count;
+    size_t cursor;
 } Editor;
 
+void editor_recompute_lines(Editor *e) {
+    size_t begin = 0;
+    for (size_t i = 0; i < e->data_count; ++i) {
+        if (e->data[i] == '\n') {
+            e->lines[e->lines_count].begin = begin;
+            e->lines[e->lines_count].end = i;
+            e->lines_count += 1;
+            begin = i + 1;
+        }
+    }
+    e->lines[e->lines_count].begin = begin;
+    e->lines[e->lines_count].end = e->data_count;
+    e->lines_count += 1;
+}
+
+size_t editor_current_line(const Editor *e) {
+    assert(e->cursor <= e->data_count);
+    for (size_t i = 0; i < e->lines_count; ++i) {
+        if (
+            e->lines[i].begin <= e->cursor 
+            && 
+            e->cursor <= e->lines[i].end
+        ) return i;
+    }
+    return 0;
+}
+
+void editor_rerender(const Editor *e) {
+    printf("\033[2J\033[H"); // clear screen and go to 0, 0
+    fwrite(e->data, 1, e->data_count, stdout);
+    size_t line = editor_current_line(e);
+    // position the cursor
+    printf(
+        "\033[%zu;%zuH", // '%zu' is a specifier for a `size_t`
+        line + 1, 
+        e->cursor - e->lines[line].begin + 1
+    );
+}
+
 static Editor editor = {0};
 
-int main(int argc, char **argv)
-{
+int editor_start_interactive(Editor *e) {
     if (!isatty(0)) {
         fprintf(stderr, "Please run in the terminal!\n");
         return 1;
     }
 
+    struct termios term; // stores the state of the terminal
+    if (tcgetattr(0, &term)) {
+        fprintf(
+            stderr, 
+            "ERROR: could not save the state of the terminal: %s\n", 
+            strerror(errno)
+        );
+        return 1;
+    }
+
+    term.c_lflag = term.c_lflag & ~ECHO; // equivalent of &= ~ECHO
+    term.c_lflag = term.c_lflag & ~ICANON;
+
+    if (tcsetattr(0, 0, &term)) {
+        fprintf(
+            stderr, 
+            "ERROR: could not update the state of the terminal: %s\n", 
+            strerror(errno)
+        );
+        return 1;
+    }
+
+    bool quit = false;
+    bool insert = false;
+    
+    while (!quit && !feof(stdin)) {
+        editor_rerender(e);
+        if (insert) {
+
+        } else {
+            int x = fgetc(stdin);
+            switch (x) {
+                case 'q': {
+                    quit = true;
+                } break;
+                case 'h': {
+                    if (e->cursor > 0) {
+                        e->cursor = e->cursor - 1;
+                    }
+                } break;
+                case 'l': {
+                    if (e->cursor < EDITOR_CAPACITY) {
+                        e->cursor += 1;
+                    }
+                } break;
+            }
+        }
+    }
+
+    printf("\033[2J");
+
+    term.c_lflag |= ECHO;
+    tcsetattr(0, 0, &term);
+    return 0;
+
+}
+
+int main(int argc, char **argv)
+{
     if (argc < 2) {
         fprintf(stderr, "Usage: zed <input.txt>\n");
         fprintf(stderr, "ERROR: no input file was provided\n");
         return 1;
     }
 
-    // stores the state of the terminal
-    struct termios term;
-    if (tcgetattr(0, &term)) {
-        fprintf(stderr, "ERROR: could not save the state of the terminal: %s\n", strerror(errno));
-        return 1;
-    }
-
-    term.c_lflag &= ~ECHO;
-    if (tcsetattr(0, 0, &term)) {
-        fprintf(stderr, "ERROR: could not update the state of the terminal: %s\n", strerror(errno));
-        return 1;
-    }
-
     const char *file_path = argv[1];
     FILE *f = fopen(file_path, "rb");
     if (f == NULL) {
@@ -54,22 +147,11 @@ int main(int argc, char **argv)
         return 1;
     }
 
-    editor.count = fread(editor.data, 1, ZED_CAPACITY, f);
-    printf("\ncontents of %s:\n\n", file_path);
-    fwrite(editor.data, 1, editor.count, stdout);
+    editor.data_count = fread(editor.data, 1, EDITOR_CAPACITY, f);
 
     fclose(f);
 
-    bool quit = false;
-    while (!quit && !feof(stdin)) {
-        int x = fgetc(stdin);
-        if (x == 'q') {
-            quit = true;
-        }
-    }
+    editor_recompute_lines(&editor);
 
-    term.c_lflag |= ECHO;
-    tcsetattr(0, 0, &term);
-
-    return 0;
+    return editor_start_interactive(&editor);
 }