diff --git a/input.txt b/input.txt
index e6a2f7b..5188976 100644
Binary files a/input.txt and b/input.txt differ
diff --git a/main.c b/main.c
index 463a80e..24acf9f 100644
--- a/main.c
+++ b/main.c
@@ -8,7 +8,7 @@
 
 #include <unistd.h>
 
-#define EDITOR_CAPACITY 1024
+#define EDITOR_CAPACITY (10*1024)
 
 typedef struct {
     size_t begin;
@@ -24,6 +24,8 @@ typedef struct {
 } Editor;
 
 void editor_compute_lines(Editor *e) {
+    // populate `e` with entries for `e->lines` and `e->data_count`
+    e->lines_count = 0;
     size_t begin = 0;
     for (size_t i = 0; i < e->data_count; ++i) {
         if (e->data[i] == '\n') {
@@ -38,6 +40,16 @@ void editor_compute_lines(Editor *e) {
     e->lines_count += 1;
 }
 
+void editor_insert_char(Editor *e, char x) {
+    if (e->data_count < EDITOR_CAPACITY) {
+        memmove(&e->data[e->cursor + 1], &e->data[e->cursor], e->data_count - e->cursor);
+        e->data[e->cursor] = x;
+        e->cursor += 1;
+        e->data_count += 1;
+        editor_compute_lines(e);
+    }
+}
+
 size_t editor_current_line(const Editor *e) {
     assert(e->cursor <= e->data_count);
     Line line;
@@ -64,7 +76,7 @@ void editor_rerender(const Editor *e) {
     );
 }
 
-int editor_start_interactive(Editor *e) {
+int editor_start_interactive(Editor *e, const char * filepath) {
     if (!isatty(0)) {
         fprintf(stderr, "Please run in the terminal!\n");
         return 1;
@@ -98,10 +110,23 @@ int editor_start_interactive(Editor *e) {
     while (!quit && !feof(stdin)) {
         editor_rerender(e);
         if (insert) {
-
+            int i = fgetc(stdin);
+            switch (i) {
+                case 27: {
+                    insert = false;
+                    FILE *f = fopen(filepath, "wb");
+                    fwrite(e->data, 1, e->data_count, f);
+                } break;
+                default: {
+                    editor_insert_char(e, i);
+                }
+            }
         } else {
             int x = fgetc(stdin);
             switch (x) {
+                case 'i': {
+                    insert = true;
+                } break;
                 case 'q': {
                     quit = true;
                 } break;
@@ -115,6 +140,26 @@ int editor_start_interactive(Editor *e) {
                         e->cursor += 1;
                     }
                 } break;
+                case 'j': {
+                    size_t current_line = editor_current_line(e);
+                    if (current_line < e->lines_count - 1) {
+                        size_t current_col = e->cursor - e->lines[current_line].begin;
+                        e->cursor = e->lines[current_line + 1].begin + current_col;
+                        if (e->cursor > e->lines[current_line + 1].end) {
+                            e->cursor = e->lines[current_line + 1].end;
+                        }
+                    }
+                } break;
+                case 'k': {
+                    size_t current_line = editor_current_line(e);
+                    if (current_line > 0) {
+                        size_t current_col = e->cursor - e->lines[current_line].begin;
+                        e->cursor = e->lines[current_line - 1].begin + current_col;
+                        if (e->cursor > e->lines[current_line - 1].end) {
+                            e->cursor = e->lines[current_line - 1].end;
+                        }
+                    }
+                } break;
             }
         }
     }
@@ -129,14 +174,14 @@ int editor_start_interactive(Editor *e) {
 
 static Editor editor;
 
-int main(int argc, char **argv) {
-    if (argc < 2) {
+int main(int num_args, char **args) {
+    if (num_args < 2) {
         fprintf(stderr, "Usage: zed <input.txt>\n");
         fprintf(stderr, "ERROR: no input file was provided\n");
         return 1;
     }
 
-    const char *file_path = argv[1];
+    const char *file_path = args[1];
     FILE *f = fopen(file_path, "rb");
     if (f == NULL) {
         fprintf(
@@ -155,6 +200,6 @@ int main(int argc, char **argv) {
     fclose(f);
     editor_compute_lines(&editor);
 
-    return editor_start_interactive(&editor);
+    return editor_start_interactive(&editor, file_path);
     return 0;
 }