Examples of Promises
What LazyImage actually is
<![CDATA[
class LazyImage
{
friend class IMAGE;
virtual void fill_in(IMAGE& im) const = 0;
protected:
card nrows, ncols, depth;
public:
LazyImage(const card _nrows, const card _ncols, const card _depth)
: nrows(_nrows), ncols(_ncols), depth(_depth) {}
};
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How to amend a recipe
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class FractalMap : public LazyImage
{
public:
FractalMap(const card order, const Seeds& _seeds = Seeds(128),
const card bits_per_pixel=8);
virtual int get_noise(const card scale) const;
private:
Seeds seeds;
void fill_in(IMAGE& im) const;
};
class GaussClouds : public FractalMap
{
public:
GaussClouds(const card order) : FractalMap(order) {}
// Well-known Gaussian random number generator
inline int get_noise(const card scale) const {...}
};
IMAGE image = GaussClouds(8);
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Lazy vectors/matrices in Numerical Math
a boon:
when a function has to compute a brand new vector/matrix and return
them
SVD svd(A);
cout << "condition number of matrix A " << svd.q_cond_number();
Vector x = SVD_inv_mult(svd,b); // Solution of Ax=b
The last function "left-divides" vector b by a (possibly non-square)
matrix A. Since the function (a constructor, actually) returns just a
promise of this division, the actual computation occurs when it's
required (while constructing a new vector in the main function). Note,
b doesn't have to be merely a vector, it can be a thick matrix, too.
An advanced example
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// Adjusting an image to a suitable size
class adjust_image : public LazyImage
{
bool do_coerce;
const IMAGE& raw_image;
void fill_in(IMAGE& im) const;
public:
adjust_image(const IMAGE& _raw_image);
};
void adjust_image::fill_in(IMAGE& im) const
{
if( do_coerce )
im.coerce(raw_image);
else
im.rectangle(rowcol(0,0),
rowcol(raw_image.q_nrows()-1,raw_image.q_ncols()-1)) =
(Rectangle)raw_image;
}
static void compress_image(const char * image_file,
const char * out_file)
{
IMAGE image(image_file);
NCLaplPyr lp(suitable_image_p(image) ? image :
(IMAGE)adjust_image(image),2);
}
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