[vlc-devel] [PATCH] clock: Cast floating point scaled timestamps back to vlc_tick_t before adding offsets

[Thomas Guillem]

LGTM.

On Wed, Mar 27, 2019, at 12:56, Martin Storsjö wrote:
> Since the absolute values of vlc_tick_t times (especially for
> system times) can be rather large, handling them in floating point
> form can lose precision.
> 
> This is a concrete issue with input_clock_t, where rate is float,
> which only stores 24 bits of precision, while a system vlc_tick_t
> measured since system boot easily exceeds 40 bits.
> 
> clock.c does similar rescalings, but the rate variables are double
> there, which only would be a problem if handling absolute timestamps
> over 53 bits.
> ---
>  src/clock/input_clock.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/src/clock/input_clock.c b/src/clock/input_clock.c
> index 3e9bfc6a60..c17a28bf72 100644
> --- a/src/clock/input_clock.c
> +++ b/src/clock/input_clock.c
> @@ -316,7 +316,7 @@ void input_clock_ChangeRate( input_clock_t *cl, 
> float rate )
>      {
>          /* Move the reference point (as if we were playing at the new 
> rate
>           * from the start */
> -        cl->ref.system = cl->last.system - (cl->last.system - 
> cl->ref.system) / rate * cl->rate;
> +        cl->ref.system = cl->last.system - (vlc_tick_t) 
> ((cl->last.system - cl->ref.system) / rate * cl->rate);
>      }
>      cl->rate = rate;
>  
> @@ -568,7 +568,7 @@ static vlc_tick_t ClockStreamToSystem( 
> input_clock_t *cl, vlc_tick_t i_stream )
>      if( !cl->b_has_reference )
>          return VLC_TICK_INVALID;
>  
> -    return ( i_stream - cl->ref.stream ) / cl->rate + cl->ref.system;
> +    return (vlc_tick_t) (( i_stream - cl->ref.stream ) / cl->rate) + 
> cl->ref.system;
>  }
>  
>  
> /*****************************************************************************
> @@ -579,7 +579,7 @@ static vlc_tick_t ClockStreamToSystem( 
> input_clock_t *cl, vlc_tick_t i_stream )
>  static vlc_tick_t ClockSystemToStream( input_clock_t *cl, vlc_tick_t 
> i_system )
>  {
>      assert( cl->b_has_reference );
> -    return ( i_system - cl->ref.system ) * cl->rate + cl->ref.stream;
> +    return (vlc_tick_t) (( i_system - cl->ref.system ) * cl->rate) + 
> cl->ref.stream;
>  }
>  
>  /**
> -- 
> 2.17.1
> 
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