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annotate mupdf-source/thirdparty/harfbuzz/docs/serializer.md @ 20:eb3dd22fef2c

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FIX: the new "sdist" build target is PHONY also
author Franz Glasner <fzglas.hg@dom66.de>
date Thu, 18 Sep 2025 22:04:13 +0200
parents b50eed0cc0ef
children
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1 # Introduction
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3 In hb-subset serialization is the process of writing the subsetted font
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4 tables out to actual bytes in the final format. All serialization works
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5 through an object called the serialize context
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6 ([hb_serialize_context_t](https://github.com/harfbuzz/harfbuzz/blob/main/src/hb-serialize.hh)).
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8 Internally the serialize context holds a fixed size memory buffer. For simple
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9 tables the final bytes are written into the buffer sequentially to produce
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10 the final serialized bytes.
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12 ## Simple Tables
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14 Simple tables are tables that do not use offset graphs.
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16 To write a struct into the serialization context, first you call an
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17 allocation method on the context which requests a writable array of bytes of
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18 a fixed size. If the requested array will not exceed the bounds of the fixed
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19 buffer the serializer will return a pointer to the next unwritten portion
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20 of the buffer. Then the struct is cast onto the returned pointer and values
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21 are written to the structs fields.
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23 Internally the serialization context ends up looking like:
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25 ```
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26 +-------+-------+-----+-------+--------------+
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27 | Obj 1 | Obj 2 | ... | Obj N | Unused Space |
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28 +-------+-------+-----+-------+--------------+
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29 ```
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31 Here Obj N, is the object currently being written.
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33 ## Complex Tables
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35 Complex tables are made up of graphs of objects, where offset's are used
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36 to form the edges of the graphs. Each object is a continuous slice of bytes
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37 that contains zero or more offsets pointing to more objects.
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39 In this case the serialization buffer has a different layout:
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41 ```
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42 |- in progress objects -| |--- packed objects --|
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43 +-----------+-----------+--------------+-------+-----+-------+
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44 | Obj n+2 | Obj n+1 | Unused Space | Obj n | ... | Obj 0 |
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45 +-----------+-----------+--------------+-------+-----+-------+
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46 |-----------------------> <---------------------|
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47 ```
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49 The buffer holds two stacks:
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51 1. In progress objects are held in a stack starting from the start of buffer
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52 that grows towards the end of the buffer.
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54 2. Packed objects are held in a stack that starts at the end of the buffer
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55 and grows towards the start of the buffer.
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57 Once the object on the top of the in progress stack is finished being written
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58 its bytes are popped from the in progress stack and copied to the top of
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59 the packed objects stack. In the example above, finalizing Obj n+1
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60 would result in the following state:
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62 ```
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63 +---------+--------------+---------+-------+-----+-------+
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64 | Obj n+2 | Unused Space | Obj n+1 | Obj n | ... | Obj 0 |
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65 +---------+--------------+---------+-------+-----+-------+
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66 ```
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68 Each packed object is associated with an ID, it's zero based position in the packed
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69 objects stack. In this example Obj 0, would have an ID of 0.
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71 During serialization offsets that link from one object to another are stored
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72 using object ids. The serialize context maintains a list of links between
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73 objects. Each link records the parent object id, the child object id, the position
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74 of the offset field within the parent object, and the width of the offset.
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76 Links are always added to the current in progress object and you can only link too
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77 objects that have been packed and thus have an ID.
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79 ### Object De-duplication
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81 An important optimization in packing offset graphs is de-duplicating equivalent objects. If you
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82 have two or more parent objects that point to child objects that are equivalent then you only need
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83 to encode the child once and can have the parents point to the same child. This can significantly
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84 reduce the final size of a serialized graph.
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86 During packing of an inprogress object the serialization context checks if any existing packed
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87 objects are equivalent to the object being packed. Here equivalence means the object has the
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88 exact same bytes and all of it's links are equivalent. If an equivalent object is found the
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89 in progress object is discarded and not copied to the packed object stack. The object id of
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90 the equivalent object is instead returned. Thus parent objects will then link to the existing
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91 equivalent object.
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93 To find equivalent objects the serialization context maintains a hashmap from object to the canonical
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94 object id.
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96 ### Link Resolution
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98 Once all objects have been packed the next step is to assign actual values to all of the offset
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99 fields. Prior to this point all links in the graph have been recorded using object id's. For each
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100 link the resolver computes the offset between the parent and child and writes the offset into
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101 the serialization buffer at the appropriate location.
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102
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103 ### Offset Overflow Resolution
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104
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105 If during link resolution the resolver finds that an offsets value would exceed what can be encoded
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106 in that offset field link resolution is aborted and the offset overflow resolver is invoked.
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107 That process is documented [here](reapcker.md).
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108
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109
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110 ### Example of Complex Serialization
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111
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112
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113 If we wanted to serialize the following graph:
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114
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115 ```
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116 a--b--d
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117 \ /
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118 c
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119 ```
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120
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121 Serializer would be called like this:
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122
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123 ```c++
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124 hb_serialize_context_t ctx;
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125
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126 struct root {
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127 char name;
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128 Offset16To<child> child_1;
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129 Offset16To<child> child_2;
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130 }
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131
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132 struct child {
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133 char name;
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134 Offset16To<char> leaf;
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135 }
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136
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137 // Object A.
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138 ctx->push();
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139 root* a = ctx->start_embed<root> ();
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140 ctx->extend_min (a);
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141 a->name = 'a';
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142
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143 // Object B.
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144 ctx->push();
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145 child* b = ctx->start_embed<child> ();
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146 ctx->extend_min (b);
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147 b->name = 'b';
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148
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149 // Object D.
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150 ctx->push();
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151 *ctx->allocate_size<char> (1) = 'd';
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152 unsigned d_id = ctx->pop_pack ();
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153
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154 ctx->add_link (b->leaf, d_id);
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155 unsigned b_id = ctx->pop_pack ();
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156
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157 // Object C
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158 ctx->push();
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159 child* c = ctx->start_embed<child> ();
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160 ctx->extend_min (c);
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161 c->name = 'c';
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162
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163 // Object D.
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164 ctx->push();
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165 *ctx->allocate_size<char> (1) = 'd';
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166 d_id = ctx->pop_pack (); // Serializer will automatically de-dup this with the previous 'd'
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167
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168 ctx->add_link (c->leaf, d_id);
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169 unsigned c_id = ctx->pop_pack ();
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170
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171 // Object A's links:
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172 ctx->add_link (a->child_1, b_id);
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173 ctx->add_link (a->child_2, c_id);
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174 ctx->pop_pack ();
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175
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176 ctx->end_serialize ();
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177
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178 ```