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@chapter Expression Evaluation
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@c man begin EXPRESSION EVALUATION
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When evaluating an arithemetic expression, Libav uses an internal
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formula evaluator, implemented through the @file{libavutil/eval.h}
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interface.
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An expression may contain unary, binary operators, constants, and
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functions.
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Two expressions @var{expr1} and @var{expr2} can be combined to form
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another expression "@var{expr1};@var{expr2}".
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@var{expr1} and @var{expr2} are evaluated in turn, and the new
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expression evaluates to the value of @var{expr2}.
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The following binary operators are available: @code{+}, @code{-},
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@code{*}, @code{/}, @code{^}.
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The following unary operators are available: @code{+}, @code{-}.
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The following functions are available:
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@table @option
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@item sinh(x)
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@item cosh(x)
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@item tanh(x)
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@item sin(x)
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@item cos(x)
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@item tan(x)
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@item atan(x)
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@item asin(x)
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@item acos(x)
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@item exp(x)
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@item log(x)
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@item abs(x)
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@item squish(x)
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@item gauss(x)
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@item isnan(x)
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Return 1.0 if @var{x} is NAN, 0.0 otherwise.
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@item mod(x, y)
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@item max(x, y)
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@item min(x, y)
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@item eq(x, y)
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@item gte(x, y)
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@item gt(x, y)
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@item lte(x, y)
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@item lt(x, y)
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@item st(var, expr)
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Allow to store the value of the expression @var{expr} in an internal
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variable. @var{var} specifies the number of the variable where to
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store the value, and it is a value ranging from 0 to 9. The function
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returns the value stored in the internal variable.
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@item ld(var)
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Allow to load the value of the internal variable with number
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@var{var}, which was previosly stored with st(@var{var}, @var{expr}).
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The function returns the loaded value.
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@item while(cond, expr)
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Evaluate expression @var{expr} while the expression @var{cond} is
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non-zero, and returns the value of the last @var{expr} evaluation, or
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NAN if @var{cond} was always false.
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@item ceil(expr)
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Round the value of expression @var{expr} upwards to the nearest
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integer. For example, "ceil(1.5)" is "2.0".
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@item floor(expr)
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Round the value of expression @var{expr} downwards to the nearest
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integer. For example, "floor(-1.5)" is "-2.0".
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@item trunc(expr)
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Round the value of expression @var{expr} towards zero to the nearest
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integer. For example, "trunc(-1.5)" is "-1.0".
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@end table
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Note that:
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@code{*} works like AND
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@code{+} works like OR
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thus
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@example
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if A then B else C
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@end example
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is equivalent to
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@example
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A*B + not(A)*C
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@end example
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When A evaluates to either 1 or 0, that is the same as
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@example
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A*B + eq(A,0)*C
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@end example
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In your C code, you can extend the list of unary and binary functions,
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and define recognized constants, so that they are available for your
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expressions.
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The evaluator also recognizes the International System number
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postfixes. If 'i' is appended after the postfix, powers of 2 are used
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instead of powers of 10. The 'B' postfix multiplies the value for 8,
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and can be appended after another postfix or used alone. This allows
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using for example 'KB', 'MiB', 'G' and 'B' as postfix.
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Follows the list of available International System postfixes, with
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indication of the corresponding powers of 10 and of 2.
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@table @option
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@item y
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-24 / -80
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@item z
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-21 / -70
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@item a
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-18 / -60
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@item f
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-15 / -50
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@item p
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-12 / -40
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@item n
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-9 / -30
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@item u
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-6 / -20
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@item m
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-3 / -10
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@item c
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-2
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@item d
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-1
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@item h
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2
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@item k
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3 / 10
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@item K
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3 / 10
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@item M
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6 / 20
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@item G
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9 / 30
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@item T
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12 / 40
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@item P
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15 / 40
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@item E
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18 / 50
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@item Z
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21 / 60
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@item Y
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24 / 70
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@end table
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@c man end