jumplist: browser-style (or 'tagstack') navigation #11530

Traditionally, when navigating to a specific location from the middle of
the jumplist results in shifting the current location to the bottom of
the list and adding the new location after it.  This behavior is not
desireable to all users--see, for example
https://vi.stackexchange.com/questions/18344/how-to-change-jumplist-behavior.

Here, another jumplist behavior is introduced.  When jumpoptions (a new
option set added here) includes stack, the jumplist behaves like the
tagstack or like history in a web browser.  That is, when navigating to
a location from the middle of the jumplist

    2 first
    1 second
    0 third <-- current location
    1 fourth
    2 fifth

to a new location the locations after the current location in the jump
list are discarded

    2 first
    1 second
    0 third
            <-- current location

The result is that when moving forward from that location, the new
location will be appended to the jumplist:

    3 first
    2 second
    1 third
    0 new

If the new location is the same

  new == second

as some previous (but not immediately prior) entry in the jumplist,

    2 first
    1 second
    0 third <-- current location
    1 fourth
    2 fifth

both occurrences preserved

    3 first
    2 second
    1 third
    0 second (new)

when moving forward from that location.

It would be desireable to go farther and, when the new location is the
same as the location that is currently next in the jumplist,

    new == fourth

make the result of navigating to the new location by jumping (e.g. 50gg)
be the same as moving forward in the jumplist

    2 first
    1 second
    0 third
    1 new <-- current location
    2 fifth

and simply increment the jumplist index.  That change is NOT part of
this patch because it would require passing the new cursor location to
the function (setpcmark) from all of its callees.  That in turn would
require those callees to know *before* calling what the new cursor
location is, which do they do not currently.

4 files changed, 43 insertions, 3 deletions

diff --git a/src/nvim/mark.c b/src/nvim/mark.c
index e5070f23ff..93bc497cf0 100644
--- a/src/nvim/mark.c
+++ b/src/nvim/mark.c
@@ -178,6 +178,16 @@ void setpcmark(void)
     curwin->w_pcmark.lnum = 1;
   }
 
+  if (jop_flags & JOP_STACK) {
+    // If we're somewhere in the middle of the jumplist discard everything
+    // after the current index.
+    if (curwin->w_jumplistidx < curwin->w_jumplistlen - 1) {
+      // Discard the rest of the jumplist by cutting the length down to
+      // contain nothing beyond the current index.
+      curwin->w_jumplistlen = curwin->w_jumplistidx + 1;
+    }
+  }
+
   /* If jumplist is full: remove oldest entry */
   if (++curwin->w_jumplistlen > JUMPLISTSIZE) {
     curwin->w_jumplistlen = JUMPLISTSIZE;
@@ -1204,7 +1214,20 @@ void cleanup_jumplist(win_T *wp, bool checktail)
         break;
       }
     }
-    if (i >= wp->w_jumplistlen) {  // no duplicate
+    bool mustfree;
+    if (i >= wp->w_jumplistlen) {  // not duplicate
+      mustfree = false;
+    } else if (i > from + 1) {  // non-adjacent duplicate
+      // When the jump options include "stack", duplicates are only removed from
+      // the jumplist when they are adjacent.
+      mustfree = !(jop_flags & JOP_STACK);
+    } else {  // adjacent duplicate
+      mustfree = true;
+    }
+
+    if (mustfree) {
+      xfree(wp->w_jumplist[from].fname);
+    } else {
       if (to != from) {
         // Not using wp->w_jumplist[to++] = wp->w_jumplist[from] because
         // this way valgrind complains about overlapping source and destination
@@ -1212,8 +1235,6 @@ void cleanup_jumplist(win_T *wp, bool checktail)
         wp->w_jumplist[to] = wp->w_jumplist[from];
       }
       to++;
-    } else {
-      xfree(wp->w_jumplist[from].fname);
     }
   }
   if (wp->w_jumplistidx == wp->w_jumplistlen) {
diff --git a/src/nvim/option.c b/src/nvim/option.c
index 68ab310329..e48ed201e8 100644
--- a/src/nvim/option.c
+++ b/src/nvim/option.c
@@ -2184,6 +2184,7 @@ static void didset_options(void)
   (void)opt_strings_flags(p_tc, p_tc_values, &tc_flags, false);
   (void)opt_strings_flags(p_ve, p_ve_values, &ve_flags, true);
   (void)opt_strings_flags(p_wop, p_wop_values, &wop_flags, true);
+  (void)opt_strings_flags(p_jop, p_jop_values, &jop_flags, true);
   (void)spell_check_msm();
   (void)spell_check_sps();
   (void)compile_cap_prog(curwin->w_s);
@@ -2632,6 +2633,10 @@ did_set_string_option(
     if (strcmp((char *)(*varp), HIGHLIGHT_INIT) != 0) {
       errmsg = e_unsupportedoption;
     }
+  } else if (varp == &p_jop) {  // 'jumpoptions'
+    if (opt_strings_flags(p_jop, p_jop_values, &jop_flags, true) != OK) {
+      errmsg = e_invarg;
+    }
   } else if (gvarp == &p_nf) {  // 'nrformats'
     if (check_opt_strings(*varp, p_nf_values, true) != OK) {
       errmsg = e_invarg;
diff --git a/src/nvim/option_defs.h b/src/nvim/option_defs.h
index c6e3f71016..4f9f32794b 100644
--- a/src/nvim/option_defs.h
+++ b/src/nvim/option_defs.h
@@ -473,6 +473,12 @@ EXTERN char_u   *p_isf;         // 'isfname'
 EXTERN char_u   *p_isi;         // 'isident'
 EXTERN char_u   *p_isp;         // 'isprint'
 EXTERN int p_js;                // 'joinspaces'
+EXTERN char_u *p_jop;           // 'jumpooptions'
+EXTERN unsigned jop_flags;
+#ifdef IN_OPTION_C
+static char *(p_jop_values[]) = { "stack", NULL };
+#endif
+#define JOP_STACK               0x01
 EXTERN char_u   *p_kp;          // 'keywordprg'
 EXTERN char_u   *p_km;          // 'keymodel'
 EXTERN char_u   *p_langmap;     // 'langmap'
diff --git a/src/nvim/options.lua b/src/nvim/options.lua
index 1e7105219f..93bfc1c0b1 100644
--- a/src/nvim/options.lua
+++ b/src/nvim/options.lua
@@ -1300,6 +1300,14 @@ return {
       defaults={if_true={vi=true}}
     },
     {
+      full_name='jumpoptions', abbreviation='jop',
+      type='string', list='onecomma', scope={'global'},
+      deny_duplicates=true,
+      varname='p_jop',
+      vim=true,
+      defaults={if_true={vim=''}}
+    },
+    {
       full_name='keymap', abbreviation='kmp',
       type='string', scope={'buffer'},
       normal_fname_chars=true,