compress: Overhaul the meaning of --aggressiveness.
[paraslash.git] / buffer_tree.h
1 /* Copyright (C) 2009 Andre Noll <maan@tuebingen.mpg.de>, see file COPYING. */
2
3 /**
4 * \file buffer_tree.h Buffer tree management.
5 *
6 * \par Buffer trees and buffer tree nodes.
7 * The buffer tree API offers a more powerful method than standard unix pipes
8 * for managing the data flow from the producer of the data (e.g. the network
9 * receiver) to its consumer(s) (e.g. a sound card).
10 *
11 * A buffer tree consists of buffer tree nodes linked via certain parent/child
12 * relationships.
13 *
14 * Each data buffer starts its way from the root of the buffer tree. At each
15 * node the data is investigated and possibly changed. New data is then fed to
16 * each child. Everything happens within one single-treaded process. There are
17 * no file descriptors and no calls to read() or write().
18 *
19 * Whenever a node in the buffer tree creates output, either by creating a new
20 * buffer or by pushing down buffers received from its parent, references to
21 * that buffer are created for all children of the node. The buffer tree code
22 * tries hard to avoid to copy buffer contents, but is forced to do so in case
23 * there are alignment constraints.
24 *
25 * Communication between nodes is possible via the btr_exec_up() function.
26 * For example, in para_audiod the alsa writer asks all parent nodes
27 * for for the number of channels and the sample rate of the current
28 * audio file.
29 *
30 * Buffer pools - An alternative to malloc/free buffer management.
31 *
32 * Non-leaf nodes usually create output to be processed by their children. The
33 * data must be fed through the output channel(s) of the node in order to make
34 * that data available to each child.
35 *
36 * The easiest way to do so is to malloc() a buffer, fill it, and then call
37 * btr_add_output(). This adds references to that buffer to all children. The
38 * buffer is automatically freed if no buffer tree node is using it any more.
39 *
40 * This approach, while being simple, has some drawbacks, especially affecting
41 * the root nodes of the buffer tree. Often the data source which is
42 * represented by a root node does not know in advance how much data will be
43 * available. Therefore the allocated buffer is either larger than what can
44 * currently be read, or is too small so that multiple buffers have to be used.
45 *
46 * While this could be worked around by using a large buffer and calling
47 * realloc() afterwards to shrink the buffer according to how much has been
48 * read, there is a second problem which comes from the alignment constraints
49 * of some filters, mainly the decoders like mp3dec. These need a minimal
50 * amount of data to proceed, and most of them even need this amount as one
51 * contiguous buffer, i.e. not spread out over two or more buffers.
52 *
53 * Although the buffer tree code handles this case just fine, it can be
54 * expensive because two or more buffers must be merged by copying buffer
55 * contents around in order to satisfy the constraint.
56 *
57 * This is where buffer pools come into play. Buffer pools try to satisfy
58 * alignment constraints without copying buffer content whenever possible. To
59 * avoid spreading out the input data over the address space like in the
60 * malloc/free approach, a fixed large contiguous buffer (the area) is used
61 * instead. A buffer pool consists basically of an area and two pointers, the
62 * read head and the write head.
63 *
64 * Once a buffer pool has been created, its node, e.g. a receiver, obtains the
65 * current value of the write head and writes new data to this location. Then
66 * it calls btr_add_output_pool() to tell much data it has written. This
67 * advances the write head accordingly, and it also creates references to the
68 * newly written part of the area for the children of the node to consume.
69 *
70 * Child nodes consume data by working through their input queue, which is a
71 * list of buffer references. Once the content of a buffer is no longer needed
72 * by a child node, the child calls btr_consume() to indicate the amount of
73 * data which can be dropped from the child's point of view. If no reference
74 * to some region of the buffer pool area remains, the read head of the buffer
75 * pool advances, making space available for the receiver node to fill.
76 *
77 * No matter if malloc() or a buffer pool is used, the buffer tree code takes
78 * care of alignment constraints imposed by the consumers. In the buffer pool
79 * case, automatic merging of references to contiguous buffers is performed.
80 * memcpy is only used if a constraint can not be satisfied by using the
81 * remaining part of the area only. This only happens when the end of the area
82 * is reached.
83 */
84
85 struct btr_node;
86 struct btr_pool;
87
88 /**
89 * The three different types of buffer tree nodes.
90 *
91 * Usually, there is exactly one node in the buffer tree, the root node, which
92 * has no parent. Every node different from the root node has exactly one
93 * parent. The root node represents a data source. Root nodes are thus used by
94 * the receivers of paraslash. Also, reading from stdin is realized as the root
95 * node of a buffer tree.
96 *
97 * Each node may have arbitrary many children, including none. Nodes with no
98 * children are called leaf nodes. They represent a data sink, like the alsa or
99 * the file writer.
100 *
101 * Hence there are three different types of buffer tree nodes: The root node
102 * and the leaf nodes and nodes which have both a parent and at least one
103 * child. Such a node is called an internal node.
104 *
105 * Internal nodes represent filters through which data buffers flow, possibly
106 * while being altered on their way to the children of the node. Examples of
107 * internal nodes are audio file decoders (mp3dec, oggdec, ...), but also the
108 * check for a wav header is implemented as an internal buffer tree node.
109 */
110 enum btr_node_type {
111 /* This node has no parent. */
112 BTR_NT_ROOT,
113 /* Node has parent and at least one child. */
114 BTR_NT_INTERNAL,
115 /* Node has no children. */
116 BTR_NT_LEAF,
117 };
118
119 /**
120 * Per node handler used for inter node communication.
121 *
122 * Each node in the buffer tree may optionally provide a command handler for
123 * execution of commands by other nodes of the tree.
124 *
125 * It is dependent on the node in question which commands are supported and how
126 * they work. In any case, the input for the command handler is some string and
127 * its output is also a string which is returned via the \a result pointer of
128 * the handler.
129 *
130 * This mechanism is used in para_audiod e.g. by the alsa writer which needs to
131 * know the sample rate of its input known to e.g. the mp3dec node further up
132 * in the buffer tree.
133 */
134 typedef int (*btr_command_handler)(struct btr_node *btrn,
135 const char *command, char **result);
136
137 /**
138 * Structure for creating new buffer tree nodes.
139 *
140 * btr_new_node() takes a pointer to such a structure.
141 *
142 * There are four different combinations of \a parent and child:
143 *
144 * 1. both \p NULL. This creates a new buffer tree with a single isolated node.
145 *
146 * 2. \a parent != \p NULL, \a child == NULL. This creates a new leaf node by
147 * adding the new node to the list of children of the given parent node.
148 *
149 * 3. \a parent == NULL, \a child != NULL. The new node becomes the new root of
150 * the buffer tree. The child must be old root.
151 *
152 * 4. both != NULL. This creates a new internal node. \a child must be child of
153 * p. This mode of operation is currently not needed and is thus not yet
154 * implemented.
155 */
156 struct btr_node_description {
157 /** Name of the new node. */
158 const char *name;
159 /** Parent of the new node. */
160 struct btr_node *parent;
161 /** Child of the new node. */
162 struct btr_node *child;
163 /** Used for inter node communication. Optional. */
164 btr_command_handler handler;
165 /** Points usually to the struct that contains the node pointer. */
166 void *context;
167 };
168
169 size_t btr_pool_size(struct btr_pool *btrp);
170 struct btr_pool *btr_pool_new(const char *name, size_t area_size);
171 void btr_pool_free(struct btr_pool *btrp);
172 size_t btr_pool_get_buffer(struct btr_pool *btrp, char **result);
173 int btr_pool_get_buffers(struct btr_pool *btrp, struct iovec iov[2]);
174 void btr_add_output_pool(struct btr_pool *btrp, size_t size,
175 struct btr_node *btrn);
176 size_t btr_pool_unused(struct btr_pool *btrp);
177 void btr_copy(const void *src, size_t n, struct btr_pool *btrp,
178 struct btr_node *btrn);
179
180 struct btr_node *btr_new_node(struct btr_node_description *bnd);
181 void btr_remove_node(struct btr_node **btrnp);
182 void btr_add_output(char *buf, size_t size, struct btr_node *btrn);
183 void btr_add_output_dont_free(const char *buf, size_t size, struct btr_node *btrn);
184 size_t btr_get_input_queue_size(struct btr_node *btrn);
185 size_t btr_get_output_queue_size(struct btr_node *btrn);
186 bool btr_no_parent(struct btr_node *btrn);
187 size_t btr_next_buffer(struct btr_node *btrn, char **bufp);
188 size_t btr_next_buffer_omit(struct btr_node *btrn, size_t omit, char **bufp);
189 void btr_consume(struct btr_node *btrn, size_t numbytes);
190 int btr_exec_up(struct btr_node *btrn, const char *command, char **value_result);
191 void btr_splice_out_node(struct btr_node **btrnp);
192 void btr_pushdown(struct btr_node *btrn);
193 void *btr_context(struct btr_node *btrn);
194 void btr_merge(struct btr_node *btrn, size_t dest_size);
195 void btr_log_tree(struct btr_node *btrn, int ll);
196 void btr_pushdown_one(struct btr_node *btrn);
197 bool btr_inplace_ok(struct btr_node *btrn);
198 int btr_node_status(struct btr_node *btrn, size_t min_iqs,
199 enum btr_node_type type);
200 void btr_get_node_start(struct btr_node *btrn, struct timeval *tv);
201 struct btr_node *btr_search_node(const char *name, struct btr_node *root);
202 void btr_drain(struct btr_node *btrn);
203 struct btr_node *btr_parent(struct btr_node *btrn);