feat: ranged candidate replace

Adds candidate attribute `replace_start`, which allows a candidate
to replace a middle part of the original text.

doc/arif_fetch.3

@@ -39,6 +39,7 @@ type is defined as:
 struct arif_cand {
     char const *text;
     int         len;
+    int         replace_start;
     int         replace_len;
     char const *transform;
     int         transform_len;
@@ -57,7 +58,8 @@ is its length in bytes.
 Member
 .I replace_len
 is the number of leading bytes of the original text that are meant to
-be replaced by the candidate.
+be replaced by the candidate, starting from byte offset
+.IR replace_start .
 .PP
 Should the original text passed to
 .BR arif_query ()

examples/arif_rime.c

@@ -227,13 +227,14 @@ copy_candidate (
     char const *text     = menu->candidates[candidate_idx].text;
     int         text_len = strlen(text);
     char const *line     = composition->preedit;
-    int         line_len = strlen(line);
+    int         line_len = composition->length;
 
     char *buf = malloc(text_len + line_len);
     assert(buf != NULL);
 
     dest->text          = memcpy(buf, text, text_len);
     dest->len           = text_len;
+    dest->replace_start = composition->sel_start;
     dest->replace_len   = composition->sel_end - composition->sel_start;
     dest->transform     = memcpy(buf + text_len, line, line_len);
     dest->transform_len = line_len;

include/arif.h

@@ -64,6 +64,7 @@ typedef int (arif_engine_query_func) (
 struct arif_cand {
     char const *text;
     int         len;
+    int         replace_start;
     int         replace_len;
     char const *transform;
     int         transform_len;

src/arif.c

@@ -131,6 +131,7 @@ copy_candidate (
     *dest = (struct arif_cand) {
         .text          = src->text,
         .len           = src->len,
+        .replace_start = src->replace_start,
         .replace_len   = src->replace_len,
         .transform     = src->transform,
         .transform_len = src->transform_len,

src/arif_rl.c

@@ -87,12 +87,19 @@ arif_rl_complete(
         char *match = malloc(sizeof(char) * (len + 1));
         assert(match != NULL);
 
-        memcpy(match, text, skip);
-        memcpy(match + skip, cand->text, cand->len);
-        memcpy(match + skip + cand->len, line + cand->replace_len,
-                len - cand->len - skip);
         match[len] = '\0';
-        comp_list[idx] = match;
+        comp_list[idx] = memcpy(match, text, skip);
+
+        int offset = skip;
+        memcpy(match + offset, line, cand->replace_start);
+
+        offset += cand->replace_start;
+        memcpy(match + offset, cand->text, cand->len);
+
+        offset += cand->len;
+        memcpy(match + offset,
+                line + cand->replace_start + cand->replace_len,
+                len - offset);
     }
     comp_list[num + 1] = NULL;