Browser viewer: Allow showing the time left in the current chapter/book in the header and footer areas of the book.

To use go to the preferences of the browser viewer and customize the
headers and footers to display the time left. Note that time left
in chapter only works correctly if chapters are in separate HTML files
in the book.

src/calibre/srv/TODO.rst

@@ -14,8 +14,6 @@ New features for the in-browser viewer
 - Allow loading fonts from the computer running calibre and using them
   for reading.
 
-- Add a time left for chapter/book footer.
-
 
 New features for the server generally
 ---------------------------------------

src/pyj/read_book/iframe.pyj

@@ -273,7 +273,7 @@ class IframeBoss:
                 if cfi != self.last_cfi:
                     self.last_cfi = cfi
                     self.send_message('update_cfi', cfi=cfi, replace_history=self.replace_history_on_next_cfi_update,
-                                      progress_frac=self.calculate_progress_frac(current_name, index))
+                                      progress_frac=self.calculate_progress_frac(current_name, index), file_progress_frac=progress_frac())
                     self.replace_history_on_next_cfi_update = True
 
     def update_toc_position(self):

src/pyj/read_book/paged_mode.pyj

@@ -466,7 +466,8 @@ def scroll_by_page(backward, by_screen):
         pos = previous_col_location() if backward else next_col_location()
         pages = 1
     if pos is -1:
-        get_boss().report_human_scroll()
+        # dont report human scroll since we dont know if a full page was
+        # scrolled or not
         get_boss().send_message('next_spine_item', previous=backward)
     else:
         if not backward:

src/pyj/read_book/prefs/head_foot.pyj

@@ -34,6 +34,9 @@ def create_item(region, label):
             opt(_('Current section'), 'section'),
             sep(),
             opt(_('Progress'), 'progress'),
+            opt(_('Time to read book'), 'time-book'),
+            opt(_('Time to read chapter'), 'time-chapter'),
+            opt(_('Time to read chapter and book'), 'time-chapter-book'),
             opt(_('Clock'), 'clock'),
         ))
     )
@@ -103,14 +106,26 @@ def commit_head_foot(onchange, container):
 
 
 if window.Intl?.DateTimeFormat:
-    date_formatter = window.Intl.DateTimeFormat(undefined, {'hour':'numeric', 'minute':'numeric'})
+    time_formatter = window.Intl.DateTimeFormat(undefined, {'hour':'numeric', 'minute':'numeric'})
 else:
-    date_formatter = {'format': def(date):
+    time_formatter = {'format': def(date):
         return '{}:{}'.format(date.getHours(), date.getMinutes())
     }
 
 
-def render_head_foot(div, which, region, progress_frac, metadata, current_toc_node, current_toc_toplevel_node):
+def format_time_left(seconds):
+    hours, minutes = divmod(int(seconds / 60), 60)
+    if hours <= 0:
+        if minutes < 1:
+            return _('almost done')
+        minutes = minutes
+        return _('{} mins').format(minutes)
+    if not minutes:
+        return _('{} hours').format(hours)
+    return _('{} h {} mins').format(hours, minutes)
+
+
+def render_head_foot(div, which, region, progress_frac, metadata, current_toc_node, current_toc_toplevel_node, book_time, chapter_time):
     template = get_session_data().get(which) or {}
     field = template[region] or 'empty'
     interface_data = get_interface_data()
@@ -128,7 +143,7 @@ def render_head_foot(div, which, region, progress_frac, metadata, current_toc_no
             ival = fmt_sidx(ival, use_roman=interface_data.use_roman_numerals_for_series_number)
             text = _('{0} of {1}').format(ival, metadata.series)
     elif field is 'clock':
-        text = date_formatter.format(Date())
+        text = time_formatter.format(Date())
         has_clock = True
     elif field is 'section':
         text = current_toc_node.title if current_toc_node else ''
@@ -136,6 +151,16 @@ def render_head_foot(div, which, region, progress_frac, metadata, current_toc_no
         text = current_toc_toplevel_node.title if current_toc_toplevel_node else ''
         if not text:
             text = current_toc_node.title if current_toc_node else ''
+    elif field.startswith('time-'):
+        if book_time is None or chapter_time is None:
+            text = _('Calculating...')
+        else:
+            if field is 'time-book':
+                text = format_time_left(book_time)
+            elif field is 'time-chapter':
+                text = format_time_left(chapter_time)
+            else:
+                text = '{} ({})'.format(format_time_left(chapter_time), format_time_left(book_time))
     if text is not div.textContent:
         div.textContent = text
     div.style.display = 'block' if text else 'none'

src/pyj/read_book/timers.pyj

@@ -31,8 +31,8 @@ class Timers:
             self.average = avg
             sq = 0
             for v'var i = 0; i < rlen; i++':
-                x = rates[i]
+                x = rates[i] - avg
-                sq += (x - avg) * (x - avg)
+                sq += x * x
             self.stddev = Math.sqrt(sq / (rlen - 1))
         else:
             self.average = self.stddev = 0
@@ -42,7 +42,7 @@ class Timers:
         self.last_scroll_at = now = window.performance.now()
         if last_scroll_at is None:
             return
-        time_since_last_scroll = now - last_scroll_at
+        time_since_last_scroll = (now - last_scroll_at) / 1000
         if time_since_last_scroll <= 0 or time_since_last_scroll >= 300:
             return
         if is_large_scroll and time_since_last_scroll < 2:
@@ -54,3 +54,8 @@ class Timers:
             self.rates.shift()
         self.rates.push(rate)
         self.calculate()
+
+    def time_for(self, length):
+        if length >= 0 and self.rates.length >= THRESHOLD and self.average > 0:
+            return length / self.average
+        return None

src/pyj/read_book/view.pyj

@@ -119,7 +119,7 @@ class View:
         self.ui = ui
         self.timers = Timers()
         self.loaded_resources = {}
-        self.current_progress_frac = 0
+        self.current_progress_frac = self.current_file_progress_frac = 0
         self.current_toc_node = self.current_toc_toplevel_node = None
         self.clock_timer_id = 0
         sd = get_session_data()
@@ -504,6 +504,7 @@ class View:
         self.ui.db.update_last_read_time(self.book)
         lrd = {'device':get_device_uuid(), 'cfi':data.cfi, 'pos_frac':data.progress_frac}
         self.current_progress_frac = data.progress_frac
+        self.current_file_progress_frac = data.file_progress_frac
         self.update_header_footer()
         key = self.book.key
         if username:
@@ -517,13 +518,24 @@ class View:
         sd = get_session_data()
         has_clock = False
 
+        if self.book?.manifest:
+            book_length = self.book.manifest.total_length or 0
+            name = self.currently_showing.name
+            chapter_length = self.book.manifest.files[name]?.length or 0
+        else:
+            book_length = chapter_length = 0
+        book_length *= max(0, 1 - self.current_progress_frac)
+        chapter_length *=  max(0, 1 - self.current_file_progress_frac)
+        book_time = self.timers.time_for(book_length)
+        chapter_time = self.timers.time_for(chapter_length)
+
         def render_template(div, sz_attr, name):
             nonlocal has_clock
             if sd.get(sz_attr, 20) > 5:
                 mi = self.book.metadata
-                texta, hca = render_head_foot(div.firstChild, name, 'left', self.current_progress_frac, mi, self.current_toc_node, self.current_toc_toplevel_node)
+                texta, hca = render_head_foot(div.firstChild, name, 'left', self.current_progress_frac, mi, self.current_toc_node, self.current_toc_toplevel_node, book_time, chapter_time)
-                textb, hcb = render_head_foot(div.firstChild.nextSibling, name, 'middle', self.current_progress_frac, mi, self.current_toc_node, self.current_toc_toplevel_node)
+                textb, hcb = render_head_foot(div.firstChild.nextSibling, name, 'middle', self.current_progress_frac, mi, self.current_toc_node, self.current_toc_toplevel_node, book_time, chapter_time)
-                textc, hcc = render_head_foot(div.lastChild, name, 'right', self.current_progress_frac, mi, self.current_toc_node, self.current_toc_toplevel_node)
+                textc, hcc = render_head_foot(div.lastChild, name, 'right', self.current_progress_frac, mi, self.current_toc_node, self.current_toc_toplevel_node, book_time, chapter_time)
                 has_clock = hca or hcb or hcc
                 if textc and not textb and not texta:
                     # Want right-aligned