feat(eval): partially port v8.2.0878

Problem:    No reduce() function.
Solution:   Add a reduce() function. (closes vim/vim#5481)
https://github.com/vim/vim/commit/85629985b71035608a37ba3bde86968481490d46

Needs CHECK_LIST_MATERIALIZE from v8.2.0751 (and range_list_materialize from
8.2.0149).

Move e_reduceempty to funcs.c, as it's only used there. Make it static.
Use tv_blob_len, tv_list_len == 0 for empty checks.
Replace vim_memset(&funcexe, 0, ...) with FUNCEXE_INIT.
Leave li initially undefined (tv_list_first returns NULL if list is NULL).

This patch has a memory leak fixed by v8.2.0882.

3 files changed, 117 insertions, 0 deletions

diff --git a/src/nvim/eval.lua b/src/nvim/eval.lua
index 18967b80f2..e00a14fca7 100644
--- a/src/nvim/eval.lua
+++ b/src/nvim/eval.lua
@@ -276,6 +276,7 @@ return {
     range={args={1, 3}, base=1},
     readdir={args={1, 2}, base=1},
     readfile={args={1, 3}, base=1},
+    reduce={args={2, 3}, base=1},
     reg_executing={},
     reg_recording={},
     reg_recorded={},
diff --git a/src/nvim/eval/funcs.c b/src/nvim/eval/funcs.c
index 29619f62e9..1a5f6e08bc 100644
--- a/src/nvim/eval/funcs.c
+++ b/src/nvim/eval/funcs.c
@@ -100,6 +100,7 @@ PRAGMA_DIAG_POP
 static char *e_listarg = N_("E686: Argument of %s must be a List");
 static char *e_listblobarg = N_("E899: Argument of %s must be a List or Blob");
 static char *e_invalwindow = N_("E957: Invalid window number");
+static char *e_reduceempty = N_("E998: Reduce of an empty %s with no initial value");
 
 /// Dummy va_list for passing to vim_snprintf
 ///
@@ -7855,6 +7856,90 @@ static void f_reverse(typval_T *argvars, typval_T *rettv, FunPtr fptr)
   }
 }
 
+/// "reduce(list, { accumlator, element -> value } [, initial])" function
+static void f_reduce(typval_T *argvars, typval_T *rettv, FunPtr fptr)
+{
+  if (argvars[0].v_type != VAR_LIST && argvars[0].v_type != VAR_BLOB) {
+    emsg(_(e_listblobreq));
+    return;
+  }
+
+  const char_u *func_name;
+  partial_T *partial = NULL;
+  if (argvars[1].v_type == VAR_FUNC) {
+    func_name = argvars[1].vval.v_string;
+  } else if (argvars[1].v_type == VAR_PARTIAL) {
+    partial = argvars[1].vval.v_partial;
+    func_name = partial_name(partial);
+  } else {
+    func_name = (const char_u *)tv_get_string(&argvars[1]);
+  }
+  if (*func_name == NUL) {
+    return;  // type error or empty name
+  }
+
+  funcexe_T funcexe = FUNCEXE_INIT;
+  funcexe.evaluate = true;
+  funcexe.partial = partial;
+
+  typval_T accum;
+  typval_T argv[3];
+  if (argvars[0].v_type == VAR_LIST) {
+    const list_T *const l = argvars[0].vval.v_list;
+    const listitem_T *li;
+
+    if (argvars[2].v_type == VAR_UNKNOWN) {
+      if (tv_list_len(l) == 0) {
+        semsg(_(e_reduceempty), "List");
+        return;
+      }
+      const listitem_T *const first = tv_list_first(l);
+      accum = *TV_LIST_ITEM_TV(first);
+      li = TV_LIST_ITEM_NEXT(l, first);
+    } else {
+      accum = argvars[2];
+      li = tv_list_first(l);
+    }
+
+    tv_copy(&accum, rettv);
+    for (; li != NULL; li = TV_LIST_ITEM_NEXT(l, li)) {
+      argv[0] = accum;
+      argv[1] = *TV_LIST_ITEM_TV(li);
+      if (call_func(func_name, -1, rettv, 2, argv, &funcexe) == FAIL) {
+        return;
+      }
+      accum = *rettv;
+    }
+  } else {
+    const blob_T *const b = argvars[0].vval.v_blob;
+    int i;
+
+    if (argvars[2].v_type == VAR_UNKNOWN) {
+      if (tv_blob_len(b) == 0) {
+        semsg(_(e_reduceempty), "Blob");
+        return;
+      }
+      accum.v_type = VAR_NUMBER;
+      accum.vval.v_number = tv_blob_get(b, 0);
+      i = 1;
+    } else {
+      accum = argvars[2];
+      i = 0;
+    }
+
+    tv_copy(&accum, rettv);
+    for (; i < tv_blob_len(b); i++) {
+      argv[0] = accum;
+      argv[1].v_type = VAR_NUMBER;
+      argv[1].vval.v_number = tv_blob_get(b, i);
+      if (call_func(func_name, -1, rettv, 2, argv, &funcexe) == FAIL) {
+        return;
+      }
+      accum = *rettv;
+    }
+  }
+}
+
 #define SP_NOMOVE       0x01        ///< don't move cursor
 #define SP_REPEAT       0x02        ///< repeat to find outer pair
 #define SP_RETCOUNT     0x04        ///< return matchcount
diff --git a/src/nvim/testdir/test_listdict.vim b/src/nvim/testdir/test_listdict.vim
index f6c404d390..42b46fc76c 100644
--- a/src/nvim/testdir/test_listdict.vim
+++ b/src/nvim/testdir/test_listdict.vim
@@ -620,6 +620,37 @@ func Test_reverse_sort_uniq()
   call assert_fails('call reverse("")', 'E899:')
 endfunc
 
+" reduce a list or a blob
+func Test_reduce()
+  call assert_equal(1, reduce([], { acc, val -> acc + val }, 1))
+  call assert_equal(10, reduce([1, 3, 5], { acc, val -> acc + val }, 1))
+  call assert_equal(2 * (2 * ((2 * 1) + 2) + 3) + 4, reduce([2, 3, 4], { acc, val -> 2 * acc + val }, 1))
+  call assert_equal('a x y z', ['x', 'y', 'z']->reduce({ acc, val -> acc .. ' ' .. val}, 'a'))
+  call assert_equal(#{ x: 1, y: 1, z: 1 }, ['x', 'y', 'z']->reduce({ acc, val -> extend(acc, { val: 1 }) }, {}))
+  call assert_equal([0, 1, 2, 3], reduce([1, 2, 3], function('add'), [0]))
+
+  let l = ['x', 'y', 'z']
+  call assert_equal(42, reduce(l, function('get'), #{ x: #{ y: #{ z: 42 } } }))
+  call assert_equal(['x', 'y', 'z'], l)
+
+  call assert_equal(1, reduce([1], { acc, val -> acc + val }))
+  call assert_equal('x y z', reduce(['x', 'y', 'z'], { acc, val -> acc .. ' ' .. val }))
+  call assert_equal(120, range(1, 5)->reduce({ acc, val -> acc * val }))
+  call assert_fails("call reduce([], { acc, val -> acc + val })", 'E998: Reduce of an empty List with no initial value')
+
+  call assert_equal(1, reduce(0z, { acc, val -> acc + val }, 1))
+  call assert_equal(1 + 0xaf + 0xbf + 0xcf, reduce(0zAFBFCF, { acc, val -> acc + val }, 1))
+  call assert_equal(2 * (2 * 1 + 0xaf) + 0xbf, 0zAFBF->reduce({ acc, val -> 2 * acc + val }, 1))
+
+  call assert_equal(0xff, reduce(0zff, { acc, val -> acc + val }))
+  call assert_equal(2 * (2 * 0xaf + 0xbf) + 0xcf, reduce(0zAFBFCF, { acc, val -> 2 * acc + val }))
+  call assert_fails("call reduce(0z, { acc, val -> acc + val })", 'E998: Reduce of an empty Blob with no initial value')
+
+  call assert_fails("call reduce({}, { acc, val -> acc + val }, 1)", 'E897:')
+  call assert_fails("call reduce(0, { acc, val -> acc + val }, 1)", 'E897:')
+  call assert_fails("call reduce('', { acc, val -> acc + val }, 1)", 'E897:')
+endfunc
+
 " splitting a string to a List
 func Test_str_split()
   call assert_equal(['aa', 'bb'], split('  aa  bb '))