cordbscs.c

/*
 * Copyright (c) 1993-1994 by Xerox Corporation.  All rights reserved.
 *
 * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
 * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
 *
 * Permission is hereby granted to use or copy this program
 * for any purpose,  provided the above notices are retained on all copies.
 * Permission to modify the code and to distribute modified code is granted,
 * provided the above notices are retained, and a notice that the code was
 * modified is included with the above copyright notice.
 */

#ifdef HAVE_CONFIG_H
# include "config.h"
#endif
#ifndef CORD_BUILD
# define CORD_BUILD
#endif

# include "gc.h"
# include "cord.h"
# include <stdlib.h>
# include <stdio.h>
# include <string.h>

/* An implementation of the cord primitives.  These are the only        */
/* Functions that understand the representation.  We perform only       */
/* minimal checks on arguments to these functions.  Out of bounds       */
/* arguments to the iteration functions may result in client functions  */
/* invoked on garbage data.  In most cases, client functions should be  */
/* programmed defensively enough that this does not result in memory    */
/* smashes.                                                             */

typedef void (* oom_fn)(void);

oom_fn CORD_oom_fn = (oom_fn) 0;

# define OUT_OF_MEMORY {  if (CORD_oom_fn != (oom_fn) 0) (*CORD_oom_fn)(); \
                          ABORT("Out of memory"); }
# define ABORT(msg) { fprintf(stderr, "%s\n", msg); abort(); }

typedef unsigned long word;

typedef union {
    struct Concatenation {
        char null;
        char header;
        char depth;     /* concatenation nesting depth. */
        unsigned char left_len;
                        /* Length of left child if it is sufficiently   */
                        /* short; 0 otherwise.                          */
#           define MAX_LEFT_LEN 255
        word len;
        CORD left;      /* length(left) > 0     */
        CORD right;     /* length(right) > 0    */
    } concatenation;
    struct Function {
        char null;
        char header;
        char depth;     /* always 0     */
        char left_len;  /* always 0     */
        word len;
        CORD_fn fn;
        void * client_data;
    } function;
    struct Generic {
        char null;
        char header;
        char depth;
        char left_len;
        word len;
    } generic;
    char string[1];
} CordRep;

# define CONCAT_HDR 1

# define FN_HDR 4
# define SUBSTR_HDR 6
        /* Substring nodes are a special case of function nodes.        */
        /* The client_data field is known to point to a substr_args     */
        /* structure, and the function is either CORD_apply_access_fn   */
        /* or CORD_index_access_fn.                                     */

/* The following may be applied only to function and concatenation nodes: */
#define IS_CONCATENATION(s)  (((CordRep *)s)->generic.header == CONCAT_HDR)

#define IS_FUNCTION(s)  ((((CordRep *)s)->generic.header & FN_HDR) != 0)

#define IS_SUBSTR(s) (((CordRep *)s)->generic.header == SUBSTR_HDR)

#define LEN(s) (((CordRep *)s) -> generic.len)
#define DEPTH(s) (((CordRep *)s) -> generic.depth)
#define GEN_LEN(s) (CORD_IS_STRING(s) ? strlen(s) : LEN(s))

#define LEFT_LEN(c) ((c) -> left_len != 0? \
                                (c) -> left_len \
                                : (CORD_IS_STRING((c) -> left) ? \
                                        (c) -> len - GEN_LEN((c) -> right) \
                                        : LEN((c) -> left)))

#define SHORT_LIMIT (sizeof(CordRep) - 1)
        /* Cords shorter than this are C strings */


/* Dump the internal representation of x to stdout, with initial        */
/* indentation level n.                                                 */
void CORD_dump_inner(CORD x, unsigned n)
{
    register size_t i;

    for (i = 0; i < (size_t)n; i++) {
        fputs("  ", stdout);
    }
    if (x == 0) {
        fputs("NIL\n", stdout);
    } else if (CORD_IS_STRING(x)) {
        for (i = 0; i <= SHORT_LIMIT; i++) {
            if (x[i] == '\0') break;
            putchar(x[i]);
        }
        if (x[i] != '\0') fputs("...", stdout);
        putchar('\n');
    } else if (IS_CONCATENATION(x)) {
        register struct Concatenation * conc =
                                &(((CordRep *)x) -> concatenation);
        printf("Concatenation: %p (len: %d, depth: %d)\n",
               x, (int)(conc -> len), (int)(conc -> depth));
        CORD_dump_inner(conc -> left, n+1);
        CORD_dump_inner(conc -> right, n+1);
    } else /* function */{
        register struct Function * func =
                                &(((CordRep *)x) -> function);
        if (IS_SUBSTR(x)) printf("(Substring) ");
        printf("Function: %p (len: %d): ", x, (int)(func -> len));
        for (i = 0; i < 20 && i < func -> len; i++) {
            putchar((*(func -> fn))(i, func -> client_data));
        }
        if (i < func -> len) fputs("...", stdout);
        putchar('\n');
    }
}

/* Dump the internal representation of x to stdout      */
void CORD_dump(CORD x)
{
    CORD_dump_inner(x, 0);
    fflush(stdout);
}

CORD CORD_cat_char_star(CORD x, const char * y, size_t leny)
{
    register size_t result_len;
    register size_t lenx;
    register int depth;

    if (x == CORD_EMPTY) return(y);
    if (leny == 0) return(x);
    if (CORD_IS_STRING(x)) {
        lenx = strlen(x);
        result_len = lenx + leny;
        if (result_len <= SHORT_LIMIT) {
            register char * result = GC_MALLOC_ATOMIC(result_len+1);

            if (result == 0) OUT_OF_MEMORY;
            memcpy(result, x, lenx);
            memcpy(result + lenx, y, leny);
            result[result_len] = '\0';
            return((CORD) result);
        } else {
            depth = 1;
        }
    } else {
        register CORD right;
        register CORD left;
        register char * new_right;

        lenx = LEN(x);

        if (leny <= SHORT_LIMIT/2
            && IS_CONCATENATION(x)
            && CORD_IS_STRING(right = ((CordRep *)x) -> concatenation.right)) {
            size_t right_len;

            /* Merge y into right part of x. */
            if (!CORD_IS_STRING(left = ((CordRep *)x) -> concatenation.left)) {
                right_len = lenx - LEN(left);
            } else if (((CordRep *)x) -> concatenation.left_len != 0) {
                right_len = lenx - ((CordRep *)x) -> concatenation.left_len;
            } else {
                right_len = strlen(right);
            }
            result_len = right_len + leny;  /* length of new_right */
            if (result_len <= SHORT_LIMIT) {
                new_right = GC_MALLOC_ATOMIC(result_len + 1);
                if (new_right == 0) OUT_OF_MEMORY;
                memcpy(new_right, right, right_len);
                memcpy(new_right + right_len, y, leny);
                new_right[result_len] = '\0';
                y = new_right;
                leny = result_len;
                x = left;
                lenx -= right_len;
                /* Now fall through to concatenate the two pieces: */
            }
            if (CORD_IS_STRING(x)) {
                depth = 1;
            } else {
                depth = DEPTH(x) + 1;
            }
        } else {
            depth = DEPTH(x) + 1;
        }
        result_len = lenx + leny;
    }
    {
      /* The general case; lenx, result_len is known: */
        register struct Concatenation * result;

        result = GC_NEW(struct Concatenation);
        if (result == 0) OUT_OF_MEMORY;
        result->header = CONCAT_HDR;
        result->depth = depth;
        if (lenx <= MAX_LEFT_LEN)
            result->left_len = (unsigned char)lenx;
        result->len = result_len;
        result->left = x;
        result->right = y;
        if (depth >= MAX_DEPTH) {
            return(CORD_balance((CORD)result));
        } else {
            return((CORD) result);
        }
    }
}


CORD CORD_cat(CORD x, CORD y)
{
    register size_t result_len;
    register int depth;
    register size_t lenx;

    if (x == CORD_EMPTY) return(y);
    if (y == CORD_EMPTY) return(x);
    if (CORD_IS_STRING(y)) {
        return(CORD_cat_char_star(x, y, strlen(y)));
    } else if (CORD_IS_STRING(x)) {
        lenx = strlen(x);
        depth = DEPTH(y) + 1;
    } else {
        register int depthy = DEPTH(y);

        lenx = LEN(x);
        depth = DEPTH(x) + 1;
        if (depthy >= depth) depth = depthy + 1;
    }
    result_len = lenx + LEN(y);
    {
        register struct Concatenation * result;

        result = GC_NEW(struct Concatenation);
        if (result == 0) OUT_OF_MEMORY;
        result->header = CONCAT_HDR;
        result->depth = depth;
        if (lenx <= MAX_LEFT_LEN)
            result->left_len = (unsigned char)lenx;
        result->len = result_len;
        result->left = x;
        result->right = y;
        if (depth >= MAX_DEPTH) {
            return(CORD_balance((CORD)result));
        } else {
            return((CORD) result);
        }
    }
}



CORD CORD_from_fn(CORD_fn fn, void * client_data, size_t len)
{
    if (len == 0) return(0);
    if (len <= SHORT_LIMIT) {
        register char * result;
        register size_t i;
        char buf[SHORT_LIMIT+1];

        for (i = 0; i < len; i++) {
            char c = (*fn)(i, client_data);

            if (c == '\0') goto gen_case;
            buf[i] = c;
        }

        result = GC_MALLOC_ATOMIC(len+1);
        if (result == 0) OUT_OF_MEMORY;
        memcpy(result, buf, len);
        result[len] = '\0';
        return((CORD) result);
    }
  gen_case:
    {
        register struct Function * result;

        result = GC_NEW(struct Function);
        if (result == 0) OUT_OF_MEMORY;
        if (result->null != 0) ABORT("error"); /* make 'null' field used */
        result->header = FN_HDR;
        /* depth is already 0 */
        result->len = len;
        result->fn = fn;
        result->client_data = client_data;
        return((CORD) result);
    }
}

size_t CORD_len(CORD x)
{
    if (x == 0) {
        return(0);
    } else {
        return(GEN_LEN(x));
    }
}

struct substr_args {
    CordRep * sa_cord;
    size_t sa_index;
};

char CORD_index_access_fn(size_t i, void * client_data)
{
    register struct substr_args *descr = (struct substr_args *)client_data;

    return(((char *)(descr->sa_cord))[i + descr->sa_index]);
}

char CORD_apply_access_fn(size_t i, void * client_data)
{
    register struct substr_args *descr = (struct substr_args *)client_data;
    register struct Function * fn_cord = &(descr->sa_cord->function);

    return((*(fn_cord->fn))(i + descr->sa_index, fn_cord->client_data));
}

/* A version of CORD_substr that simply returns a function node, thus   */
/* postponing its work. The fourth argument is a function that may      */
/* be used for efficient access to the ith character.                   */
/* Assumes i >= 0 and i + n < length(x).                                */
CORD CORD_substr_closure(CORD x, size_t i, size_t n, CORD_fn f)
{
    register struct substr_args * sa = GC_NEW(struct substr_args);
    CORD result;

    if (sa == 0) OUT_OF_MEMORY;
    sa->sa_cord = (CordRep *)x;
    sa->sa_index = i;
    result = CORD_from_fn(f, (void *)sa, n);
    if (result == CORD_EMPTY) return CORD_EMPTY; /* n == 0 */
    ((CordRep *)result) -> function.header = SUBSTR_HDR;
    return (result);
}

# define SUBSTR_LIMIT (10 * SHORT_LIMIT)
        /* Substrings of function nodes and flat strings shorter than   */
        /* this are flat strings.  Othewise we use a functional         */
        /* representation, which is significantly slower to access.     */

/* A version of CORD_substr that assumes i >= 0, n > 0, and i + n < length(x).*/
CORD CORD_substr_checked(CORD x, size_t i, size_t n)
{
    if (CORD_IS_STRING(x)) {
        if (n > SUBSTR_LIMIT) {
            return(CORD_substr_closure(x, i, n, CORD_index_access_fn));
        } else {
            register char * result = GC_MALLOC_ATOMIC(n+1);

            if (result == 0) OUT_OF_MEMORY;
            strncpy(result, x+i, n);
            result[n] = '\0';
            return(result);
        }
    } else if (IS_CONCATENATION(x)) {
        register struct Concatenation * conc
                        = &(((CordRep *)x) -> concatenation);
        register size_t left_len;
        register size_t right_len;

        left_len = LEFT_LEN(conc);
        right_len = conc -> len - left_len;
        if (i >= left_len) {
            if (n == right_len) return(conc -> right);
            return(CORD_substr_checked(conc -> right, i - left_len, n));
        } else if (i+n <= left_len) {
            if (n == left_len) return(conc -> left);
            return(CORD_substr_checked(conc -> left, i, n));
        } else {
            /* Need at least one character from each side. */
            register CORD left_part;
            register CORD right_part;
            register size_t left_part_len = left_len - i;

            if (i == 0) {
                left_part = conc -> left;
            } else {
                left_part = CORD_substr_checked(conc -> left, i, left_part_len);
            }
            if (i + n == right_len + left_len) {
                 right_part = conc -> right;
            } else {
                 right_part = CORD_substr_checked(conc -> right, 0,
                                                  n - left_part_len);
            }
            return(CORD_cat(left_part, right_part));
        }
    } else /* function */ {
        if (n > SUBSTR_LIMIT) {
            if (IS_SUBSTR(x)) {
                /* Avoid nesting substring nodes.       */
                register struct Function * f = &(((CordRep *)x) -> function);
                register struct substr_args *descr =
                                (struct substr_args *)(f -> client_data);

                return(CORD_substr_closure((CORD)descr->sa_cord,
                                           i + descr->sa_index,
                                           n, f -> fn));
            } else {
                return(CORD_substr_closure(x, i, n, CORD_apply_access_fn));
            }
        } else {
            char * result;
            register struct Function * f = &(((CordRep *)x) -> function);
            char buf[SUBSTR_LIMIT+1];
            register char * p = buf;
            register int j;
            register int lim = i + n;

            for (j = i; j < lim; j++) {
                char c = (*(f -> fn))(j, f -> client_data);

                if (c == '\0') {
                    return(CORD_substr_closure(x, i, n, CORD_apply_access_fn));
                }
                *p++ = c;
            }
            result = GC_MALLOC_ATOMIC(n+1);
            if (result == 0) OUT_OF_MEMORY;
            memcpy(result, buf, n);
            result[n] = '\0';
            return(result);
        }
    }
}

CORD CORD_substr(CORD x, size_t i, size_t n)
{
    register size_t len = CORD_len(x);

    if (i >= len || n == 0) return(0);
    if (i + n > len) n = len - i;
    return(CORD_substr_checked(x, i, n));
}

/* See cord.h for definition.  We assume i is in range. */
int CORD_iter5(CORD x, size_t i, CORD_iter_fn f1,
                         CORD_batched_iter_fn f2, void * client_data)
{
    if (x == 0) return(0);
    if (CORD_IS_STRING(x)) {
        register const char *p = x+i;

        if (*p == '\0') ABORT("2nd arg to CORD_iter5 too big");
        if (f2 != CORD_NO_FN) {
            return((*f2)(p, client_data));
        } else {
            while (*p) {
                if ((*f1)(*p, client_data)) return(1);
                p++;
            }
            return(0);
        }
    } else if (IS_CONCATENATION(x)) {
        register struct Concatenation * conc
                        = &(((CordRep *)x) -> concatenation);


        if (i > 0) {
            register size_t left_len = LEFT_LEN(conc);

            if (i >= left_len) {
                return(CORD_iter5(conc -> right, i - left_len, f1, f2,
                                  client_data));
            }
        }
        if (CORD_iter5(conc -> left, i, f1, f2, client_data)) {
            return(1);
        }
        return(CORD_iter5(conc -> right, 0, f1, f2, client_data));
    } else /* function */ {
        register struct Function * f = &(((CordRep *)x) -> function);
        register size_t j;
        register size_t lim = f -> len;

        for (j = i; j < lim; j++) {
            if ((*f1)((*(f -> fn))(j, f -> client_data), client_data)) {
                return(1);
            }
        }
        return(0);
    }
}

#undef CORD_iter
int CORD_iter(CORD x, CORD_iter_fn f1, void * client_data)
{
    return(CORD_iter5(x, 0, f1, CORD_NO_FN, client_data));
}

int CORD_riter4(CORD x, size_t i, CORD_iter_fn f1, void * client_data)
{
    if (x == 0) return(0);
    if (CORD_IS_STRING(x)) {
        register const char *p = x + i;

        for(;;) {
            char c = *p;

            if (c == '\0') ABORT("2nd arg to CORD_riter4 too big");
            if ((*f1)(c, client_data)) return(1);
            if (p == x) break;
            p--;
        }
        return(0);
    } else if (IS_CONCATENATION(x)) {
        register struct Concatenation * conc
                        = &(((CordRep *)x) -> concatenation);
        register CORD left_part = conc -> left;
        register size_t left_len;

        left_len = LEFT_LEN(conc);
        if (i >= left_len) {
            if (CORD_riter4(conc -> right, i - left_len, f1, client_data)) {
                return(1);
            }
            return(CORD_riter4(left_part, left_len - 1, f1, client_data));
        } else {
            return(CORD_riter4(left_part, i, f1, client_data));
        }
    } else /* function */ {
        register struct Function * f = &(((CordRep *)x) -> function);
        register size_t j;

        for (j = i; ; j--) {
            if ((*f1)((*(f -> fn))(j, f -> client_data), client_data)) {
                return(1);
            }
            if (j == 0) return(0);
        }
    }
}

int CORD_riter(CORD x, CORD_iter_fn f1, void * client_data)
{
    size_t len = CORD_len(x);
    if (len == 0) return(0);
    return(CORD_riter4(x, len - 1, f1, client_data));
}

/*
 * The following functions are concerned with balancing cords.
 * Strategy:
 * Scan the cord from left to right, keeping the cord scanned so far
 * as a forest of balanced trees of exponentially decreasing length.
 * When a new subtree needs to be added to the forest, we concatenate all
 * shorter ones to the new tree in the appropriate order, and then insert
 * the result into the forest.
 * Crucial invariants:
 * 1. The concatenation of the forest (in decreasing order) with the
 *     unscanned part of the rope is equal to the rope being balanced.
 * 2. All trees in the forest are balanced.
 * 3. forest[i] has depth at most i.
 */

typedef struct {
    CORD c;
    size_t len;         /* Actual length of c   */
} ForestElement;

static size_t min_len [ MAX_DEPTH ];

static int min_len_init = 0;

int CORD_max_len;

typedef ForestElement Forest [ MAX_DEPTH ];
                        /* forest[i].len >= fib(i+1)            */
                        /* The string is the concatenation      */
                        /* of the forest in order of DECREASING */
                        /* indices.                             */

void CORD_init_min_len(void)
{
    register int i;
    size_t last, previous;

    min_len[0] = previous = 1;
    min_len[1] = last = 2;
    for (i = 2; i < MAX_DEPTH; i++) {
        size_t current = last + previous;

        if (current < last) /* overflow */ current = last;
        min_len[i] = current;
        previous = last;
        last = current;
    }
    CORD_max_len = last - 1;
    min_len_init = 1;
}


void CORD_init_forest(ForestElement * forest, size_t max_len)
{
    register int i;

    for (i = 0; i < MAX_DEPTH; i++) {
        forest[i].c = 0;
        if (min_len[i] > max_len) return;
    }
    ABORT("Cord too long");
}

/* Add a leaf to the appropriate level in the forest, cleaning          */
/* out lower levels as necessary.                                       */
/* Also works if x is a balanced tree of concatenations; however        */
/* in this case an extra concatenation node may be inserted above x;    */
/* This node should not be counted in the statement of the invariants.  */
void CORD_add_forest(ForestElement * forest, CORD x, size_t len)
{
    register int i = 0;
    register CORD sum = CORD_EMPTY;
    register size_t sum_len = 0;

    while (len > min_len[i + 1]) {
        if (forest[i].c != 0) {
            sum = CORD_cat(forest[i].c, sum);
            sum_len += forest[i].len;
            forest[i].c = 0;
        }
        i++;
    }
    /* Sum has depth at most 1 greter than what would be required       */
    /* for balance.                                                     */
    sum = CORD_cat(sum, x);
    sum_len += len;
    /* If x was a leaf, then sum is now balanced.  To see this          */
    /* consider the two cases in which forest[i-1] either is or is      */
    /* not empty.                                                       */
    while (sum_len >= min_len[i]) {
        if (forest[i].c != 0) {
            sum = CORD_cat(forest[i].c, sum);
            sum_len += forest[i].len;
            /* This is again balanced, since sum was balanced, and has  */
            /* allowable depth that differs from i by at most 1.        */
            forest[i].c = 0;
        }
        i++;
    }
    i--;
    forest[i].c = sum;
    forest[i].len = sum_len;
}

CORD CORD_concat_forest(ForestElement * forest, size_t expected_len)
{
    register int i = 0;
    CORD sum = 0;
    size_t sum_len = 0;

    while (sum_len != expected_len) {
        if (forest[i].c != 0) {
            sum = CORD_cat(forest[i].c, sum);
            sum_len += forest[i].len;
        }
        i++;
    }
    return(sum);
}

/* Insert the frontier of x into forest.  Balanced subtrees are */
/* treated as leaves.  This potentially adds one to the depth   */
/* of the final tree.                                           */
void CORD_balance_insert(CORD x, size_t len, ForestElement * forest)
{
    register int depth;

    if (CORD_IS_STRING(x)) {
        CORD_add_forest(forest, x, len);
    } else if (IS_CONCATENATION(x)
               && ((depth = DEPTH(x)) >= MAX_DEPTH
                   || len < min_len[depth])) {
        register struct Concatenation * conc
                        = &(((CordRep *)x) -> concatenation);
        size_t left_len = LEFT_LEN(conc);

        CORD_balance_insert(conc -> left, left_len, forest);
        CORD_balance_insert(conc -> right, len - left_len, forest);
    } else /* function or balanced */ {
        CORD_add_forest(forest, x, len);
    }
}


CORD CORD_balance(CORD x)
{
    Forest forest;
    register size_t len;

    if (x == 0) return(0);
    if (CORD_IS_STRING(x)) return(x);
    if (!min_len_init) CORD_init_min_len();
    len = LEN(x);
    CORD_init_forest(forest, len);
    CORD_balance_insert(x, len, forest);
    return(CORD_concat_forest(forest, len));
}


/* Position primitives  */

/* Private routines to deal with the hard cases only: */

/* P contains a prefix of the  path to cur_pos. Extend it to a full     */
/* path and set up leaf info.                                           */
/* Return 0 if past the end of cord, 1 o.w.                             */
void CORD__extend_path(register CORD_pos p)
{
     register struct CORD_pe * current_pe = &(p[0].path[p[0].path_len]);
     register CORD top = current_pe -> pe_cord;
     register size_t pos = p[0].cur_pos;
     register size_t top_pos = current_pe -> pe_start_pos;
     register size_t top_len = GEN_LEN(top);

     /* Fill in the rest of the path. */
       while(!CORD_IS_STRING(top) && IS_CONCATENATION(top)) {
         register struct Concatenation * conc =
                        &(((CordRep *)top) -> concatenation);
         register size_t left_len;

         left_len = LEFT_LEN(conc);
         current_pe++;
         if (pos >= top_pos + left_len) {
             current_pe -> pe_cord = top = conc -> right;
             current_pe -> pe_start_pos = top_pos = top_pos + left_len;
             top_len -= left_len;
         } else {
             current_pe -> pe_cord = top = conc -> left;
             current_pe -> pe_start_pos = top_pos;
             top_len = left_len;
         }
         p[0].path_len++;
       }
     /* Fill in leaf description for fast access. */
       if (CORD_IS_STRING(top)) {
         p[0].cur_leaf = top;
         p[0].cur_start = top_pos;
         p[0].cur_end = top_pos + top_len;
       } else {
         p[0].cur_end = 0;
       }
       if (pos >= top_pos + top_len) p[0].path_len = CORD_POS_INVALID;
}

char CORD__pos_fetch(register CORD_pos p)
{
    /* Leaf is a function node */
    struct CORD_pe * pe = &((p)[0].path[(p)[0].path_len]);
    CORD leaf = pe -> pe_cord;
    register struct Function * f = &(((CordRep *)leaf) -> function);

    if (!IS_FUNCTION(leaf)) ABORT("CORD_pos_fetch: bad leaf");
    return ((*(f -> fn))(p[0].cur_pos - pe -> pe_start_pos, f -> client_data));
}

void CORD__next(register CORD_pos p)
{
    register size_t cur_pos = p[0].cur_pos + 1;
    register struct CORD_pe * current_pe = &((p)[0].path[(p)[0].path_len]);
    register CORD leaf = current_pe -> pe_cord;

    /* Leaf is not a string or we're at end of leaf */
    p[0].cur_pos = cur_pos;
    if (!CORD_IS_STRING(leaf)) {
        /* Function leaf        */
        register struct Function * f = &(((CordRep *)leaf) -> function);
        register size_t start_pos = current_pe -> pe_start_pos;
        register size_t end_pos = start_pos + f -> len;

        if (cur_pos < end_pos) {
          /* Fill cache and return. */
            register size_t i;
            register size_t limit = cur_pos + FUNCTION_BUF_SZ;
            register CORD_fn fn = f -> fn;
            register void * client_data = f -> client_data;

            if (limit > end_pos) {
                limit = end_pos;
            }
            for (i = cur_pos; i < limit; i++) {
                p[0].function_buf[i - cur_pos] =
                        (*fn)(i - start_pos, client_data);
            }
            p[0].cur_start = cur_pos;
            p[0].cur_leaf = p[0].function_buf;
            p[0].cur_end = limit;
            return;
        }
    }
    /* End of leaf      */
    /* Pop the stack until we find two concatenation nodes with the     */
    /* same start position: this implies we were in left part.          */
    {
        while (p[0].path_len > 0
               && current_pe[0].pe_start_pos != current_pe[-1].pe_start_pos) {
            p[0].path_len--;
            current_pe--;
        }
        if (p[0].path_len == 0) {
            p[0].path_len = CORD_POS_INVALID;
            return;
        }
    }
    p[0].path_len--;
    CORD__extend_path(p);
}

void CORD__prev(register CORD_pos p)
{
    register struct CORD_pe * pe = &(p[0].path[p[0].path_len]);

    if (p[0].cur_pos == 0) {
        p[0].path_len = CORD_POS_INVALID;
        return;
    }
    p[0].cur_pos--;
    if (p[0].cur_pos >= pe -> pe_start_pos) return;

    /* Beginning of leaf        */

    /* Pop the stack until we find two concatenation nodes with the     */
    /* different start position: this implies we were in right part.    */
    {
        register struct CORD_pe * current_pe = &((p)[0].path[(p)[0].path_len]);

        while (p[0].path_len > 0
               && current_pe[0].pe_start_pos == current_pe[-1].pe_start_pos) {
            p[0].path_len--;
            current_pe--;
        }
    }
    p[0].path_len--;
    CORD__extend_path(p);
}

#undef CORD_pos_fetch
#undef CORD_next
#undef CORD_prev
#undef CORD_pos_to_index
#undef CORD_pos_to_cord
#undef CORD_pos_valid

char CORD_pos_fetch(register CORD_pos p)
{
    if (p[0].cur_start <= p[0].cur_pos && p[0].cur_pos < p[0].cur_end) {
        return(p[0].cur_leaf[p[0].cur_pos - p[0].cur_start]);
    } else {
        return(CORD__pos_fetch(p));
    }
}

void CORD_next(CORD_pos p)
{
    if (p[0].cur_pos < p[0].cur_end - 1) {
        p[0].cur_pos++;
    } else {
        CORD__next(p);
    }
}

void CORD_prev(CORD_pos p)
{
    if (p[0].cur_end != 0 && p[0].cur_pos > p[0].cur_start) {
        p[0].cur_pos--;
    } else {
        CORD__prev(p);
    }
}

size_t CORD_pos_to_index(CORD_pos p)
{
    return(p[0].cur_pos);
}

CORD CORD_pos_to_cord(CORD_pos p)
{
    return(p[0].path[0].pe_cord);
}

int CORD_pos_valid(CORD_pos p)
{
    return(p[0].path_len != CORD_POS_INVALID);
}

void CORD_set_pos(CORD_pos p, CORD x, size_t i)
{
    if (x == CORD_EMPTY) {
        p[0].path_len = CORD_POS_INVALID;
        return;
    }
    p[0].path[0].pe_cord = x;
    p[0].path[0].pe_start_pos = 0;
    p[0].path_len = 0;
    p[0].cur_pos = i;
    CORD__extend_path(p);
}