fix(colitur-x): detect the CSV header instead of assuming it

Skipping the first line unconditionally punished the obvious way to view
one month --

    colitur emit --format csv ... | grep '^2027-04' | colitur-x

-- by silently eating 1 April, because grep had already removed the
header the program still expected. A filter is exactly what this program
should compose with.

A header is the line beginning 'date,'; anything else is a day. Verified
all three ways: full year with header 365 rows from 01-01, a grepped
month without header 30 rows from 04-01, and the old head -1 incantation
still 30 rows.

3 files changed, 21 insertions, 4 deletions

diff --git a/README.md b/README.md
index b0872cf..9df7b8e 100644
--- a/README.md
+++ b/README.md
@@ -12,8 +12,12 @@ Keys: arrows or `hjkl` move, `n`/`p` change month, `q` quits.
 It reads CSV on **stdin** and nothing else, so the shell decides what it
 shows. One month:
 
-    colitur emit --format csv --from 2027 --to 2027 \
-      | { head -1; grep "^2027-04"; } | colitur-x
+    colitur emit --format csv --from 2027 --to 2027 | grep "^2027-04" | colitur-x
+
+The header line is detected rather than assumed, so filtering it away is
+fine. An earlier version skipped the first line unconditionally, which meant
+that pipeline silently dropped 1 April -- the program fought the obvious
+filter instead of composing with it.
 
 Another language, or a local calendar:
 
diff --git a/colitur-x b/colitur-x
index f2b36f7..b5db32a 100755
--- a/colitur-x
+++ b/colitur-x
diff --git a/colitur-x.c b/colitur-x.c
index def42fa..06f2f1a 100644
--- a/colitur-x.c
+++ b/colitur-x.c
@@ -55,15 +55,28 @@ static int split(char *line, char **out)
 	return n;
 }
 
+/* The header line is DETECTED, not assumed. Skipping the first line
+   unconditionally punishes the obvious way to look at one month --
+   `... | grep "^2027-04" | colitur-x` -- by silently eating the 1st,
+   which is a filter this program should compose with rather than fight.
+   A header is exactly the line beginning "date,"; anything else is a day. */
+static int is_header(const char *line)
+{
+	return strncmp(line, "date,", 5) == 0;
+}
+
 static void load(void)
 {
 	char buf[4096];
-	if (!fgets(buf, sizeof buf, stdin)) return;      /* header */
+	int first = 1;
 	while (nrows < MAXDAYS && fgets(buf, sizeof buf, stdin)) {
 		buf[strcspn(buf, "\n")] = 0;
+		if (first && is_header(buf)) { first = 0; continue; }
+		first = 0;
+		if (buf[0] == 0) continue;
 		char *dup = strdup(buf);
 		if (!dup) break;
-		if (split(dup, rows[nrows].f) >= 12) nrows++;
+		if (split(dup, rows[nrows].f) >= 12) nrows++; else free(dup);
 	}
 }