3 files changed, 19 insertions, 2 deletions
diff --git a/text.c b/text.c
index 28115fd..2f3ea17 100644
--- a/text.c
+++ b/text.c
@@ -120,6 +120,7 @@ struct Text {
 	int fd;                 /* the file descriptor of the original mmap-ed data */
 	LineCache lines;        /* mapping between absolute pos in bytes and logical line breaks */
 	const char *marks[26];  /* a mark is a pointer into an underlying buffer */
+	int newlines;           /* 0: unknown, 1: \n, -1: \r\n */
 };
 
 /* buffer management */
@@ -832,6 +833,20 @@ bool text_modified(Text *txt) {
 	return txt->saved_action != txt->undo;
 }
 
+bool text_newlines_crnl(Text *txt){
+	if (txt->newlines == 0) {
+		txt->newlines = 1; /* default to UNIX style \n new lines */
+		const char *start = txt->buf.data;
+		if (start) {
+			const char *nl = memchr(start, '\n', txt->buf.len);
+			if (nl > start && nl[-1] == '\r')
+				txt->newlines = -1; /* Windows style \r\n */
+		}
+	}
+
+	return txt->newlines < 0;
+}
+
 static bool text_iterator_init(Iterator *it, size_t pos, Piece *p, size_t off) {
 	*it = (Iterator){
 		.pos = pos,
diff --git a/text.h b/text.h
index 15d0ad1..773e8b6 100644
--- a/text.h
+++ b/text.h
@@ -76,6 +76,8 @@ void text_mark_clear_all(Text*);
 
 size_t text_size(Text*);
 bool text_modified(Text*);
+/* test whether the underlying file uses UNIX style \n or Windows style \r\n newlines */
+bool text_newlines_crnl(Text*);
 int text_save(Text*, const char *file);
 void text_free(Text*);
 
diff --git a/vis.c b/vis.c
index d65b45b..c075dc5 100644
--- a/vis.c
+++ b/vis.c
@@ -848,8 +848,8 @@ static void insert_tab(const Arg *arg) {
 }
 
 static void insert_newline(const Arg *arg) {
-	// TODO determine file type to insert \r\n or \n
-	insert(&(const Arg){ .s = "\n" });
+	insert(&(const Arg){ .s =
+	       text_newlines_crnl(vis->win->text) ? "\r\n" : "\n" });
 }
 
 static void openline(const Arg *arg) {