奥村先生がptexでマジックコメント使えないかなぁとか言っていたのを受けて遊んでみた．（あくまで遊びで，あればいいと思っているわけじゃない．）

--- texk_orig/ptexenc/ptexenc.c 2015-10-25 10:16:06.000000000 +0100
+++ texk/ptexenc/ptexenc.c 2016-01-24 16:53:00.166659600 +0100
@@ -550,7 +550,7 @@
 
 static struct unget_st {
     int size;
-    int buff[4];
+    int buff[100];
 } ungetbuff[NOFILE];
 
 static int getc4(FILE *fp)
@@ -607,7 +607,7 @@
 {
     struct unget_st *p = &ungetbuff[fileno(fp)];
 
-    if (p->size >= 4) return EOF;
+    if (p->size >= 100) return EOF;
     return p->buff[p->size++] = c;
 }
 
@@ -752,6 +752,35 @@
     return true;
 }
 
+#define MAGICK_LEN 10
+static int read_magick_comment(FILE *fp)
+{
+    int magick[MAGICK_LEN] = {'%',' ','c','o','d','i','n','g',':',' '};
+    char c[100];
+    int i;
+    int enc;
+    for (i=0; i<MAGICK_LEN; i++) {
+        c[i] = getc4(fp);
+        if (c[i] != magick[i]) {
+            do { ungetc4(c[i], fp); } while (--i>=0);
+            return ENC_UNKNOWN;
+        }
+    }
+    for (i=MAGICK_LEN; i<100; i++) {
+        c[i] = getc4(fp);
+        if (c[i] == '\r' || c[i] == '\n' || c[i] == 0) {
+            break;
+        }
+    }
+    if (c[i] != 0) ungetc4(c[i], fp);
+    c[i] = '\0';
+    --i;
+    enc = string_to_enc(c + MAGICK_LEN);
+    do { ungetc4(c[i], fp); } while (--i>=0);
+    if (enc > 0) return enc;
+    else return ENC_UNKNOWN;
+}
+
 static int infile_enc[NOFILE]; /* ENC_UNKNOWN (=0): not determined
                                   other: determined */
 
@@ -766,7 +795,9 @@
     if (infile_enc[fd] == ENC_UNKNOWN) { /* just after opened */
         ungetbuff[fd].size = 0;
         if (isUTF8Nstream(fp)) infile_enc[fd] = ENC_UTF8;
-        else                   infile_enc[fd] = get_file_enc();
+        else                   infile_enc[fd] = read_magick_comment(fp);
+        
+        if (infile_enc[fd] == ENC_UNKNOWN) infile_enc[fd] = get_file_enc();
     }
     buffer = buff;
     first = last = pos;