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+#!/usr/bin/env python
+r"""General-purpose bisector
+
+Prints a space-separated list of values to stdout:
+1_if_success_0_otherwise left_x left_f(x) right_x right_f(x)
+
+Usage examples:
+
+# Finding the square root of 200 via bisection:
+bisector --var=BB --range=0.0,100.0 --target=200 --maxiter=100 \
+         --atol_val=1e-12 --rtol_val=0 --cmd='echo "$BB * $BB" | bc'
+# => 1 14.142135623730923 199.99999999999923 14.142135623731633 200.0000000000193
+
+# Finding an integer approximation to sqrt(200) via bisection:
+bisector --var=BB --range=0,100 --target=200 --maxiter=100 \
+         --atol_arg=1 --cmd='echo "$BB * $BB" | bc'
+# => 1 14 196.0 15 225.0
+
+# Finding a change-id that broke something via bisection:
+bisector --var=BB --range=0,1000000 --target=0.5 --maxiter=100 \
+         --atol_arg=1 \
+         --cmd='test $BB -gt 123456 && echo 1 || echo 0' --verbosity=3
+# => 1 123456 0.0 123457 1.0
+
+# Finding settings that compress /usr/share/dict/words to a given target size:
+bisector --var=BB --range=1,9 --target=250000 --atol_arg=1 \
+  --cmd='gzip -$BB </usr/share/dict/words >/tmp/w_$BB.gz; wc -c /tmp/w_$BB.gz' \
+  --final='mv /tmp/w_$BB.gz /tmp/words.gz; rm /tmp/w_*.gz' \
+  --verbosity=1
+# => 1 3 263170.0 4 240043.0
+
+# JXL-encoding with bisection-for-size (tolerance 0.5%):
+bisector --var=BB --range=0.1,3.0 --target=3500 --rtol_val=0.005 \
+  --cmd='(build/tools/cjxl --distance=$BB /tmp/baseball.png /tmp/baseball_$BB.jxl && wc -c /tmp/baseball_$BB.jxl)' \
+  --final='mv /tmp/baseball_$BB.jxl /tmp/baseball.jxl; rm -f /tmp/baseball_*.jxl' \
+  --verbosity=1
+# => 1 1.1875 3573.0 1.278125 3481.0
+
+# JXL-encoding with bisection-for-bits-per-pixel (tolerance 0.5%), using helper:
+bisector --var=BB --range=0.1,3.0 --target=1.2 --rtol_val=0.005 \
+  --cmd='(build/tools/cjxl --distance=$BB /tmp/baseball.png /tmp/baseball_$BB.jxl && get_bpp /tmp/baseball_$BB.jxl)' \
+  --final='mv /tmp/baseball_$BB.jxl /tmp/baseball.jxl; rm -f /tmp/baseball_*.jxl' \
+  --verbosity=1
+# => ...
+"""
+
+import argparse
+import os
+import re
+import subprocess
+import sys
+
+
+def _expandvars(vardef, env,
+                max_recursion=100,
+                max_length=10**6,
+                verbosity=0):
+  """os.path.expandvars() variant using parameter env rather than os.environ."""
+  current_expanded = vardef
+  for num_recursions in range(max_recursion):
+    if verbosity >= 3:
+      print(f'_expandvars(): num_recursions={num_recursions}, '
+            f'len={len(current_expanded)}' +
+            (', current: ' + current_expanded if verbosity >= 4 else ''))
+    if len > max_length:
+        break
+    current_expanded, num_replacements = re.subn(
+        r'$\{(\w+)\}|$(\w+)',
+        lambda m: env.get(m[1] if m[1] is not None else m[2], ''),
+        current_expanded)
+    if num_replacements == 0:
+        break
+  return current_expanded
+
+
+def _strtod(string):
+  """Extracts leftmost float from string (like strtod(3))."""
+  match = re.match(r'[+-]?\d*[.]?\d*(?:[eE][+-]?\d+)?', string)
+  return float(match[0]) if match[0] else None
+
+  
+def run_shell_command(shell_command,
+                      bisect_var, bisect_val,
+                      extra_env_defs,
+                      verbosity=0):
+  """Runs a shell command with env modifications, fetching return value."""
+  shell_env = dict(os.environ)
+  shell_env[bisect_var] = str(bisect_val)
+  for env_def in extra_env_defs:
+    varname, vardef = env_def.split('=', 1)
+    shell_env[varname] = _expandvars(vardev, shell_env,
+                                     verbosity=verbosity)
+  shell_ret = subprocess.run(shell_command,
+                             # We explicitly want subshell semantics!
+                             shell=True,
+                             capture_output=True,
+                             env=shell_env)
+  stdout = shell_ret.stdout.decode('utf-8')
+  score = _strtod(stdout)
+  if verbosity >= 2:
+    print(f'{bisect_var}={bisect_val} {shell_command} => '
+          f'{shell_ret.returncode} # {stdout.strip()}')
+  return (shell_ret.returncode == 0,  # Command was successful?
+          score)
+
+
+def _bisect(*,
+            shell_command,
+            final_shell_command,
+            target,
+            int_args,            
+            bisect_var, bisect_left, bisect_right,
+            rtol_val, atol_val, rtol_arg, atol_arg,
+            maxiter,
+            extra_env_defs,
+            verbosity=0
+            ):
+  """Performs bisection."""
+  def _get_val(x):
+    success, val = run_shell_command(shell_command,
+                                     bisect_var, x,
+                                     extra_env_defs,
+                                     verbosity=verbosity)
+    if not success:
+      raise RuntimeError(f'Bisection failed for: {bisect_var}={x}: '
+                         f'success={success}, val={val}, '
+                         f'cmd={shell_command}, var={bisect_var}')
+    return val
+  #
+  bisect_mid, value_mid = None, None
+  try:
+    value_left = _get_val(bisect_left)
+    value_right = _get_val(bisect_right)
+    if (value_left < target) != (target <= value_right):
+      raise RuntimeError(
+          f'Cannot bisect: target={target}, value_left={value_left}, '
+          f'value_right={value_right}')
+    for num_iter in range(maxiter):
+      bisect_mid_f = 0.5 * (bisect_left + bisect_right)
+      bisect_mid = round(bisect_mid_f) if int_args else bisect_mid_f
+      value_mid = _get_val(bisect_mid)
+      if (value_left < target) == (value_mid < target):
+        # Relative to target, `value_mid` is on the same side
+        # as `value_left`.
+        bisect_left = bisect_mid
+        value_left = value_mid
+      else:
+        # Otherwise, this situation must hold for value_right
+        # ("tertium non datur").
+        bisect_right = bisect_mid
+        value_right = value_mid
+      if verbosity >= 1:
+        print(f'bisect target={target}, '
+              f'left: {value_left} at {bisect_left}, '
+              f'right: {value_right} at {bisect_right}, '
+              f'mid: {value_mid} at {bisect_mid}')
+      delta_val = target - value_mid
+      if abs(delta_val) <= atol_val + rtol_val * abs(target):
+        return 1, bisect_left, value_left, bisect_right, value_right
+      delta_arg = bisect_right - bisect_left
+      # Also check whether the argument is "within tolerance".
+      # Here, we have to be careful if bisect_left and bisect_right
+      # have different signs: Then, their absolute magnitude
+      # "sets the relevant scale".
+      if abs(delta_arg) <= atol_arg + (
+              rtol_arg * 0.5 * (abs(bisect_left) + abs(bisect_right))):
+        return 1, bisect_left, value_left, bisect_right, value_right
+    return 0, bisect_left, value_left, bisect_right, value_right
+  finally:
+    # If cleanup is specified, always run it
+    if final_shell_command:
+        run_shell_command(
+            final_shell_command,
+            bisect_var,
+            bisect_mid if bisect_mid is not None else bisect_left,
+            extra_env_defs, verbosity=verbosity)
+
+
+def main(args):
+  """Main entry point."""
+  parser = argparse.ArgumentParser(description='mhtml_walk args')
+  parser.add_argument(
+      '--var',
+      help='The variable to use for bisection.',
+      default='BISECT')
+  parser.add_argument(
+      '--range',
+      help=('The argument range for bisecting, as {low},{high}. '
+            'If no argument has a decimal dot, assume integer parameters.'),
+      default='0.0,1.0')
+  parser.add_argument(
+      '--max',
+      help='The maximal value for bisecting.',
+      type=float,
+      default=0.0)
+  parser.add_argument(
+      '--target',
+      help='The target value to aim for.',
+      type=float,
+      default=1.0)
+  parser.add_argument(
+      '--maxiter',
+      help='The maximal number of iterations to perform.',
+      type=int,
+      default=40)
+  parser.add_argument(
+      '--rtol_val',
+      help='Relative tolerance to accept for deviations from target value.',
+      type=float,
+      default=0.0)
+  parser.add_argument(
+      '--atol_val',
+      help='Absolute tolerance to accept for deviations from target value.',
+      type=float,
+      default=0.0)
+  parser.add_argument(
+      '--rtol_arg',
+      help='Relative tolerance to accept for the argument.',
+      type=float,
+      default=0.0)
+  parser.add_argument(
+      '--atol_arg',
+      help=('Absolute tolerance to accept for the argument '
+            '(e.g. for bisecting change-IDs).'),
+      type=float,
+      default=0.0)
+  parser.add_argument(
+      '--verbosity',
+      help='The verbosity level.',
+      type=int,
+      default=1)
+  parser.add_argument(
+      '--env',
+      help=('Comma-separated list of extra environment variables '
+            'to incrementally add before executing the shell-command.'),
+      default='')
+  parser.add_argument(
+      '--cmd',
+      help=('The shell command to execute. Must print a numerical result '
+            'to stdout.'))
+  parser.add_argument(
+      '--final',
+      help='The cleanup shell command to execute.')
+  #
+  parsed = parser.parse_args(args)
+  extra_env_defs = tuple(filter(None, parsed.env.split(',')))    
+  try:
+    low_high = parsed.range.split(',')
+    if len(low_high) != 2:
+      raise ValueError('--range must be {low},{high}')
+    int_args = False
+    low_val, high_val = map(float, low_high)
+    low_val_int = round(low_val)
+    high_val_int = round(high_val)
+    if low_high == [str(low_val_int), str(high_val_int)]:
+        int_args = True
+        low_val = low_val_int
+        high_val = high_val_int
+    ret = _bisect(
+        shell_command=parsed.cmd,
+        final_shell_command=parsed.final,
+        target=parsed.target,
+        int_args=int_args,        
+        bisect_var=parsed.var,
+        bisect_left=low_val,
+        bisect_right=high_val,
+        rtol_val=parsed.rtol_val,
+        atol_val=parsed.atol_val,
+        rtol_arg=parsed.rtol_arg,
+        atol_arg=parsed.atol_arg,
+        maxiter=parsed.maxiter,
+        extra_env_defs=extra_env_defs,
+        verbosity=parsed.verbosity,
+    )
+    print(' '.join(map(str, ret)))
+  except Exception as exn:
+    sys.exit(f'Problem: {exn}')
+
+
+if __name__ == '__main__':
+  main(sys.argv[1:])