From c80f4af866508cf2812ba030bf7abfb70d126b6e Mon Sep 17 00:00:00 2001
From: Zhineng Li <im@zhineng.li>
Date: Fri, 29 May 2026 11:52:36 +0800
Subject: ch01-21: replace blanks by proper tabs

---
 ch01/ex21-entab.c | 89 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 89 insertions(+)
 create mode 100644 ch01/ex21-entab.c

(limited to 'ch01/ex21-entab.c')

diff --git a/ch01/ex21-entab.c b/ch01/ex21-entab.c
new file mode 100644
index 0000000..1282c61
--- /dev/null
+++ b/ch01/ex21-entab.c
@@ -0,0 +1,89 @@
+#include <stdio.h>
+
+#define TABSTOP 8  /* number of columns between tab stops */
+#define DEBUG   0  /* display tabs and spaces explicitly */
+
+#define SYM_SP  '.'  /* visible space for debugging */
+#define SYM_TAB '~'  /* visible tab for debugging */
+
+void puttab(void);
+void putspace(int len);
+
+
+/* replace blanks by proper tabs */
+int main()
+{
+    int c;       /* current character */
+    int offset;  /* position inside current tab stop */
+    int nspace;  /* number of spaces */
+
+    if (DEBUG) {
+        printf("# debug mode is on\n");
+        printf("# displaying spaces and tabs explicitly\n");
+        printf("# space -> %c\n", SYM_SP);
+        printf("# tab   -> %c\n", SYM_TAB);
+    }
+
+    offset = nspace = 0;
+    while ((c = getchar()) != EOF) {
+        ++offset;
+
+        if (c == ' ')
+            ++nspace;
+        else if (c == '\t') {
+            puttab();
+            offset = nspace = 0;
+        } else if (nspace) {
+            putspace(nspace);
+            nspace = 0;
+        }
+
+        if (offset == TABSTOP) {
+            if (nspace)
+                puttab();
+            offset = nspace = 0;
+        }
+
+        if (c == '\n') {
+            if (nspace)
+                putspace(nspace);
+            offset = nspace = 0;
+        }
+
+        if (c != ' ' && c != '\t')
+            putchar(c);
+    }
+
+    if (nspace)
+        putspace(nspace);
+
+    return 0;
+}
+
+/* print a tab */
+void puttab(void)
+{
+    int c;
+
+    if (DEBUG)
+        c = SYM_TAB;
+    else
+        c = '\t';
+
+    putchar(c);
+}
+
+/* print len spaces */
+void putspace(int len)
+{
+    int i;
+    int c;
+
+    if (DEBUG)
+        c = SYM_SP;
+    else
+        c = ' ';
+
+    for (i = 0; i < len; ++i)
+        putchar(c);
+}
-- 
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