/*
 * 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 <stdlib.h>
# include <stdio.h>
# include <string.h>

#include "gc.h"
#include "gc/cord.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(); }

    struct Concatenation {
        CORD left;      /* length(left) > 0     */
        CORD right;     /* length(right) > 0    */
    };

    struct Function {
        CORD_fn fn;
        void * client_data;
    };

    struct Generic {
        char nul;
        char header;
        char depth;     /* Concatenation nesting depth; 0 for function. */
        unsigned char left_len;
                        /* Length of left concatenated child if it is   */
                        /* sufficiently short; 0 otherwise.             */
        unsigned long len;
    };

union ConcatOrFunc {
    struct Concatenation concat;
    struct Function function;
};

typedef struct {
  struct Generic generic;
  union ConcatOrFunc data;
} 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 MAX_LEFT_LEN 255

#define LEFT_LEN(s) (((CordRep *)s) -> generic.left_len != 0 ? \
                ((CordRep *)s) -> generic.left_len \
                : (CORD_IS_STRING(((CordRep *)s) -> data.concat.left) ? \
                        ((CordRep *)s) -> generic.len - \
                            GEN_LEN(((CordRep *)s) -> data.concat.right) \
                        : LEN(((CordRep *)s) -> data.concat.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)
{
    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)) {
        struct Concatenation * conc = &(((CordRep *)x) -> data.concat);

        printf("Concatenation: %p (len: %d, depth: %d)\n",
               (void *)x, (int)LEN(x), (int)DEPTH(x));
        CORD_dump_inner(conc -> left, n+1);
        CORD_dump_inner(conc -> right, n+1);
    } else /* function */ {
        struct Function * f = &(((CordRep *)x) -> data.function);
        size_t lim = (size_t)LEN(x);

        if (IS_SUBSTR(x)) printf("(Substring) ");
        printf("Function: %p (len: %d): ", (void *)x, (int)lim);
        for (i = 0; i < 20 && i < lim; i++) {
            putchar((*(f -> fn))(i, f -> client_data));
        }
        if (i < lim) 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)
{
    size_t result_len;
    size_t lenx;
    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) {
            char * result = (char *)GC_MALLOC_ATOMIC(result_len + 1);

            if (NULL == result) OUT_OF_MEMORY;
#           ifdef LINT2
                memcpy(result, x, lenx + 1);
#           else
                memcpy(result, x, lenx);
                                /* No need to copy the terminating zero */
                                /* as result[lenx] is written below.    */
#           endif
            memcpy(result + lenx, y, leny);
            result[result_len] = '\0';
            return (CORD)result;
        } else {
            depth = 1;
        }
    } else {
        CORD right;
        CORD left;
        char * new_right;

        lenx = LEN(x);

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

            /* Merge y into right part of x. */
            if (!CORD_IS_STRING(left = ((CordRep *)x) -> data.concat.left)) {
                right_len = lenx - LEN(left);
            } else if (((CordRep *)x) -> generic.left_len != 0) {
                right_len = lenx - ((CordRep *)x) -> generic.left_len;
            } else {
                right_len = strlen(right);
            }
            result_len = right_len + leny;  /* length of new_right */
            if (result_len <= SHORT_LIMIT) {
                new_right = (char *)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: */
        CordRep *result = GC_NEW(CordRep);

        if (NULL == result) OUT_OF_MEMORY;
        result -> generic.header = CONCAT_HDR;
        result -> generic.depth = (char)depth;
        if (lenx <= MAX_LEFT_LEN)
            result -> generic.left_len = (unsigned char)lenx;
        result -> generic.len = (unsigned long)result_len;
        result -> data.concat.left = x;
        GC_PTR_STORE_AND_DIRTY((void *)(&result -> data.concat.right), y);
        GC_reachable_here(x);
        if (depth >= MAX_DEPTH) {
            return CORD_balance((CORD)result);
        } else {
            return (CORD)result;
        }
    }
}

CORD CORD_cat(CORD x, CORD y)
{
    size_t result_len;
    int depth;
    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 {
        int depthy = DEPTH(y);

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

        if (NULL == result) OUT_OF_MEMORY;
        result -> generic.header = CONCAT_HDR;
        result -> generic.depth = (char)depth;
        if (lenx <= MAX_LEFT_LEN)
            result -> generic.left_len = (unsigned char)lenx;
        result -> generic.len = (unsigned long)result_len;
        result -> data.concat.left = x;
        GC_PTR_STORE_AND_DIRTY((void *)&(result -> data.concat.right), y);
        GC_reachable_here(x);
        if (depth >= MAX_DEPTH) {
            return CORD_balance((CORD)result);
        } else {
            return (CORD)result;
        }
    }
}

static CordRep *CORD_from_fn_inner(CORD_fn fn, void * client_data, size_t len)
{
    if (0 == len) return NULL;
    if (len <= SHORT_LIMIT) {
        char * result;
        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 = (char *)GC_MALLOC_ATOMIC(len + 1);
        if (NULL == result) OUT_OF_MEMORY;
        memcpy(result, buf, len);
        result[len] = '\0';
        return (CordRep *)result;
    }
  gen_case:
    {
        CordRep *result = GC_NEW(CordRep);

        if (NULL == result) OUT_OF_MEMORY;
        result -> generic.header = FN_HDR;
        /* depth is already 0 */
        result -> generic.len = (unsigned long)len;
        result -> data.function.fn = fn;
        GC_PTR_STORE_AND_DIRTY(&(result -> data.function.client_data),
                               client_data);
        return result;
    }
}

CORD CORD_from_fn(CORD_fn fn, void * client_data, size_t len)
{
    return (/* const */ CORD) CORD_from_fn_inner(fn, client_data, len);
}

size_t CORD_len(CORD x)
{
    return x == 0 ? 0 : GEN_LEN(x);
}

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

char CORD_index_access_fn(size_t i, void * client_data)
{
    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)
{
    struct substr_args *descr = (struct substr_args *)client_data;
    struct Function * fn_cord = &(descr -> sa_cord -> data.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)
{
    struct substr_args * sa = GC_NEW(struct substr_args);
    CordRep * result;

    if (sa == 0) OUT_OF_MEMORY;
    sa->sa_index = i;
    GC_PTR_STORE_AND_DIRTY(&sa->sa_cord, x);
    result = CORD_from_fn_inner(f, (void *)sa, n);
    if ((CORD)result != CORD_EMPTY && 0 == result -> generic.nul)
        result -> generic.header = SUBSTR_HDR;
    return (CORD)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, 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 {
            char * result = (char *)GC_MALLOC_ATOMIC(n + 1);

            if (NULL == result) OUT_OF_MEMORY;
            strncpy(result, x+i, n);
            result[n] = '\0';
            return result;
        }
    } else if (IS_CONCATENATION(x)) {
        struct Concatenation * conc = &(((CordRep *)x) -> data.concat);
        size_t left_len = LEFT_LEN(x);
        size_t right_len = (size_t)LEN(x) - 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. */
            CORD left_part;
            CORD right_part;
            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.       */
                struct Function * f = &(((CordRep *)x) -> data.function);
                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;
            struct Function * f = &(((CordRep *)x) -> data.function);
            char buf[SUBSTR_LIMIT+1];
            char * p = buf;
            size_t j;
            size_t 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 = (char *)GC_MALLOC_ATOMIC(n + 1);
            if (NULL == result) OUT_OF_MEMORY;
            memcpy(result, buf, n);
            result[n] = '\0';
            return result;
        }
    }
}

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

    if (i >= len || 0 == n) 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 (0 == x) return 0;
    if (CORD_IS_STRING(x)) {
        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)) {
        struct Concatenation * conc = &(((CordRep *)x) -> data.concat);

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

            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 */ {
        struct Function * f = &(((CordRep *)x) -> data.function);
        size_t j;
        size_t lim = (size_t)LEN(x);

        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 (0 == x) return 0;
    if (CORD_IS_STRING(x)) {
        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--;
        }
    } else if (IS_CONCATENATION(x)) {
        struct Concatenation * conc = &(((CordRep *)x) -> data.concat);
        CORD left_part = conc -> left;
        size_t left_len = LEFT_LEN(x);

        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 */ {
        struct Function * f = &(((CordRep *)x) -> data.function);
        size_t j;

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

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

void CORD_init_forest(ForestElement * forest, size_t max_len)
{
    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)
{
    int i = 0;
    CORD sum = CORD_EMPTY;
    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)
{
    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)
{
    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])) {
        struct Concatenation * conc = &(((CordRep *)x) -> data.concat);
        size_t left_len = LEFT_LEN(x);

        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;
    size_t len;

    if (0 == x) 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(CORD_pos p)
{
     struct CORD_pe * current_pe = &(p[0].path[p[0].path_len]);
     CORD top = current_pe -> pe_cord;
     size_t pos = p[0].cur_pos;
     size_t top_pos = current_pe -> pe_start_pos;
     size_t top_len = GEN_LEN(top);

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

         left_len = LEFT_LEN(top);
         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(CORD_pos p)
{
    /* Leaf is a function node */
    struct CORD_pe * pe;
    CORD leaf;
    struct Function * f;

    if (!CORD_pos_valid(p))
        ABORT("CORD_pos_fetch: invalid argument");
    pe = &p[0].path[p[0].path_len];
    leaf = pe -> pe_cord;
    if (!IS_FUNCTION(leaf))
        ABORT("CORD_pos_fetch: bad leaf");
    f = &((CordRep *)leaf) -> data.function;
    return f -> fn(p[0].cur_pos - pe -> pe_start_pos, f -> client_data);
}

void CORD__next(CORD_pos p)
{
    size_t cur_pos = p[0].cur_pos + 1;
    struct CORD_pe * current_pe;
    CORD leaf;

    if (!CORD_pos_valid(p))
        ABORT("CORD_next: invalid argument");
    current_pe = &p[0].path[p[0].path_len];
    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        */
        struct Function * f = &(((CordRep *)leaf) -> data.function);
        size_t start_pos = current_pe -> pe_start_pos;
        size_t end_pos = start_pos + (size_t)LEN(leaf);

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

            if (end_pos - cur_pos < FUNCTION_BUF_SZ) {
                limit = end_pos - cur_pos;
            }
            for (i = 0; i < limit; i++) {
                p[0].function_buf[i] = (*fn)(i + cur_pos - start_pos,
                                             client_data);
            }
            p[0].cur_start = cur_pos;
            p[0].cur_leaf = p[0].function_buf;
            p[0].cur_end = cur_pos + 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(CORD_pos p)
{
    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.    */
    {
        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(CORD_pos p)
{
    if (p[0].cur_end != 0) {
        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 + 1 < p[0].cur_end) {
        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);
}