一个GLSL Shader的格式化算法(LALR解析器)
在进行OpenGL程序开发时，我需要自行解析`string`类型的Shader代码，抽取出里面的某些变量名和subroutine名。
由于找不到可用的GLSL Shader解析器，就照着虎书（《现代编译原理-c语言描述》）自己写了个LALR Generator，实际上包含了（词法分析器+语法分析器+格式化框架）的（LR(0)、SLR(1)、LALR(1)、LR(1)）全自动生成，支持yacc的优先级指令和Shift/Reduce、Reduce/Reduc冲突解决功能。
本文的GLSL Shader的格式化工具（下载链接在这里https://files.cnblogs.com/files/bitzhuwei/GLSL.Formatter.net8.0.rar?t=1735122490&download=true ），就是用我的LALR Generator，根据GLSL4.60.8的文法生成的，如下所述。
GLSL4.60.8的文法
#extractor <GLSL4.60.8.st.ext>translation_unit :external_declaration| translation_unit external_declaration;
external_declaration :function_definition| declaration| ';';
function_definition :function_prototype compound_statement/* compound_statement_no_new_scope */;
variable_identifier :'identifier';
primary_expression :variable_identifier| 'intConstant'| 'uintConstant'| 'floatConstant'| 'boolConstant'| 'doubleConstant'| '(' expression ')';
postfix_expression :primary_expression| postfix_expression '[' integer_expression ']'| function_call| postfix_expression '.' 'identifier' // FIELD_SELECTION| postfix_expression '++'| postfix_expression '--';
integer_expression :expression;
function_call :function_call_or_method;
function_call_or_method :function_call_generic;
function_call_generic :function_call_header_with_parameters ')'| function_call_header_no_parameters ')';
function_call_header_no_parameters :function_call_header 'void'| function_call_header;
function_call_header_with_parameters :function_call_header assignment_expression| function_call_header_with_parameters ',' assignment_expression;
function_call_header :function_identifier '(';
function_identifier :type_specifier| postfix_expression;
unary_expression :postfix_expression| '++' unary_expression| '--' unary_expression| unary_operator unary_expression;
unary_operator :'+'| '-'| '!'| '~';
multiplicative_expression :unary_expression| multiplicative_expression '*' unary_expression| multiplicative_expression '/' unary_expression| multiplicative_expression '%' unary_expression;
additive_expression :multiplicative_expression| additive_expression '+' multiplicative_expression| additive_expression '-' multiplicative_expression;
shift_expression :additive_expression| shift_expression '<<' additive_expression| shift_expression '>>' additive_expression;
relational_expression :shift_expression| relational_expression '<' shift_expression| relational_expression '>' shift_expression| relational_expression '<=' shift_expression| relational_expression '>=' shift_expression;
equality_expression :relational_expression| equality_expression '==' relational_expression| equality_expression '!=' relational_expression;
and_expression :equality_expression| and_expression '&' equality_expression;
exclusive_or_expression :and_expression| exclusive_or_expression '^' and_expression;
inclusive_or_expression :exclusive_or_expression| inclusive_or_expression '|' exclusive_or_expression;
logical_and_expression :inclusive_or_expression| logical_and_expression '&&' inclusive_or_expression;
logical_xor_expression :logical_and_expression| logical_xor_expression '^^' logical_and_expression;
logical_or_expression :logical_xor_expression| logical_or_expression '||' logical_xor_expression;
conditional_expression :logical_or_expression| logical_or_expression '?' expression ':' assignment_expression;
assignment_expression :conditional_expression| unary_expression assignment_operator assignment_expression;
assignment_operator :'='| '*='| '/='| '%='| '+='| '-='| '<<='| '>>='| '&='| '^='| '|=';
expression :assignment_expression| expression ',' assignment_expression;
constant_expression :conditional_expression;
declaration :function_prototype ';'| init_declarator_list ';'| 'precision' precision_qualifier type_specifier ';'| type_qualifier 'identifier' '{' struct_declaration_list '}' ';'| type_qualifier 'identifier' '{' struct_declaration_list '}' 'identifier' ';'| type_qualifier 'identifier' '{' struct_declaration_list '}' 'identifier' array_specifier ';'| type_qualifier ';'| type_qualifier 'identifier' ';'| type_qualifier 'identifier' identifier_list ';';
identifier_list :',' 'identifier'| identifier_list ',' 'identifier';
function_prototype :function_declarator ')';
function_declarator :function_header| function_header_with_parameters;
function_header_with_parameters :function_header parameter_declaration| function_header_with_parameters ',' parameter_declaration;
function_header :fully_specified_type 'identifier' '(';
parameter_declarator :type_specifier 'identifier'| type_specifier 'identifier' array_specifier;
parameter_declaration :type_qualifier parameter_declarator| parameter_declarator| type_qualifier parameter_type_specifier| parameter_type_specifier;
parameter_type_specifier :type_specifier;
init_declarator_list :single_declaration| init_declarator_list ',' 'identifier'| init_declarator_list ',' 'identifier' array_specifier| init_declarator_list ',' 'identifier' array_specifier '=' initializer| init_declarator_list ',' 'identifier' '=' initializer;
single_declaration :fully_specified_type| fully_specified_type 'identifier'| fully_specified_type 'identifier' array_specifier| fully_specified_type 'identifier' array_specifier '=' initializer| fully_specified_type 'identifier' '=' initializer;
fully_specified_type :type_specifier| type_qualifier type_specifier;
invariant_qualifier :'invariant';
interpolation_qualifier :'smooth'| 'flat'| 'noperspective';
layout_qualifier :'layout' '(' layout_qualifier_id_list ')';
layout_qualifier_id_list :layout_qualifier_id| layout_qualifier_id_list ',' layout_qualifier_id;
layout_qualifier_id :'identifier'| 'identifier' '=' constant_expression| 'shared';
precise_qualifier :'precise';
type_qualifier :single_type_qualifier| type_qualifier single_type_qualifier;
single_type_qualifier :storage_qualifier| layout_qualifier| precision_qualifier| interpolation_qualifier| invariant_qualifier| precise_qualifier;
storage_qualifier :'const'| 'in'| 'out'| 'inout'| 'centroid'| 'patch'| 'sample'| 'uniform'| 'buffer'| 'shared'| 'coherent'| 'volatile'| 'restrict'| 'readonly'| 'writeonly'| 'subroutine'| 'subroutine' '(' type_name_list ')';
type_name_list :'type_name'| type_name_list ',' 'type_name';
type_specifier :type_specifier_nonarray| type_specifier_nonarray array_specifier;
array_specifier :'[' ']'| '[' conditional_expression ']'| array_specifier '[' ']'| array_specifier '[' conditional_expression ']';
type_specifier_nonarray :'void'| 'float'| 'double'| 'int'| 'uint'| 'bool'| 'vec2'| 'vec3'| 'vec4'| 'dvec2'| 'dvec3'| 'dvec4'| 'bvec2'| 'bvec3'| 'bvec4'| 'ivec2'| 'ivec3'| 'ivec4'| 'uvec2'| 'uvec3'| 'uvec4'| 'mat2'| 'mat3'| 'mat4'| 'mat2x2'| 'mat2x3'| 'mat2x4'| 'mat3x2'| 'mat3x3'| 'mat3x4'| 'mat4x2'| 'mat4x3'| 'mat4x4'| 'dmat2'| 'dmat3'| 'dmat4'| 'dmat2x2'| 'dmat2x3'| 'dmat2x4'| 'dmat3x2'| 'dmat3x3'| 'dmat3x4'| 'dmat4x2'| 'dmat4x3'| 'dmat4x4'| 'atomic_uint'| 'sampler2D'| 'sampler3D'| 'samplerCube'| 'sampler2DShadow'| 'samplerCubeShadow'| 'sampler2DArray'| 'sampler2DArrayShadow'| 'samplerCubeArray'| 'samplerCubeArrayShadow'| 'isampler2D'| 'isampler3D'| 'isamplerCube'| 'isampler2DArray'| 'isamplerCubeArray'| 'usampler2D'| 'usampler3D'| 'usamplerCube'| 'usampler2DArray'| 'usamplerCubeArray'| 'sampler1D'| 'sampler1DShadow'| 'sampler1DArray'| 'sampler1DArrayShadow'| 'isampler1D'| 'isampler1DArray'| 'usampler1D'| 'usampler1DArray'| 'sampler2DRect'| 'sampler2DRectShadow'| 'isampler2DRect'| 'usampler2DRect'| 'samplerBuffer'| 'isamplerBuffer'| 'usamplerBuffer'| 'sampler2DMS'| 'isampler2DMS'| 'usampler2DMS'| 'sampler2DMSArray'| 'isampler2DMSArray'| 'usampler2DMSArray'| 'image2D'| 'iimage2D'| 'uimage2D'| 'image3D'| 'iimage3D'| 'uimage3D'| 'imageCube'| 'iimageCube'| 'uimageCube'| 'imageBuffer'| 'iimageBuffer'| 'uimageBuffer'| 'image1D'| 'iimage1D'| 'uimage1D'| 'image1DArray'| 'iimage1DArray'| 'uimage1DArray'| 'image2DRect'| 'iimage2DRect'| 'uimage2DRect'| 'image2DArray'| 'iimage2DArray'| 'uimage2DArray'| 'imageCubeArray'| 'iimageCubeArray'| 'uimageCubeArray'| 'image2DMS'| 'iimage2DMS'| 'uimage2DMS'| 'image2DMSArray'| 'iimage2DMSArray'| 'uimage2DMSArray'| struct_specifier| 'type_name';
precision_qualifier :'highp'| 'mediump'| 'lowp';
struct_specifier :'struct' 'type_name'/* 'identifier' */ '{' struct_declaration_list '}'| 'struct' '{' struct_declaration_list '}';
struct_declaration_list :struct_declaration| struct_declaration_list struct_declaration;
struct_declaration :type_specifier struct_declarator_list ';'| type_qualifier type_specifier struct_declarator_list ';';
struct_declarator_list :struct_declarator| struct_declarator_list ',' struct_declarator;
struct_declarator :'identifier'| 'identifier' array_specifier;
initializer :assignment_expression| '{' initializer_list '}'| '{' initializer_list ',' '}';
initializer_list :initializer| initializer_list ',' initializer;
declaration_statement :declaration;
statement :compound_statement| simple_statement;
simple_statement :declaration_statement| expression_statement| selection_statement| switch_statement| case_label| iteration_statement| jump_statement;
compound_statement :'{' '}'| '{' statement_list '}';/* merge into statement
statement_no_new_scope :compound_statement_no_new_scope| simple_statement;*//* merge into compound_statement
compound_statement_no_new_scope :'{' '}'| '{' statement_list '}';*/
statement_list :statement| statement_list statement;
expression_statement :';'| expression ';';
selection_statement :'if' '(' expression ')' selection_rest_statement;
selection_rest_statement :statement 'else' statement| statement;
condition :expression| fully_specified_type 'identifier' '=' initializer;
switch_statement :'switch' '(' expression ')' '{' switch_statement_list '}';
switch_statement_list :empty| statement_list;
case_label :'case' expression ':'| 'default' ':';
iteration_statement :'while' '(' condition ')' statement/* statement_no_new_scope */| 'do' statement 'while' '(' expression ')' ';'| 'for' '(' for_init_statement for_rest_statement ')' statement/* statement_no_new_scope */;
for_init_statement :expression_statement| declaration_statement;
conditionopt :condition| empty;
for_rest_statement :conditionopt ';'| conditionopt ';' expression;
jump_statement :'continue' ';'| 'break' ';'| 'return' ';'| 'return' expression ';'| 'discard' ';';// lexical statements// no need : 'struct' 'identifier' '{' struct_declaration_list '}'
// now I changed it into 'struct' 'type_name' '{' struct_declaration_list '}'
// only identifier next to 'struct' is a user-defined type and should be a 'type_name' token.
'type_name' %%<'struct'>[a-zA-Z_][a-zA-Z0-9_]*%%'intConstant' %%[-+]?[0-9]+%%
'intConstant' %%0x[0-9A-Fa-f]+%%
'uintConstant' %%[-+]?[0-9]+[uU]%%
'uintConstant' %%0x[0-9A-Fa-f]+[uU]%%
'floatConstant' %%[-+]?[0-9]+([.][0-9]+)?([Ee][-+]?[0-9]+)?[fF]%%
'boolConstant' %%true/[^a-zA-Z0-9_]%%
'boolConstant' %%false/[^a-zA-Z0-9_]%%
'doubleConstant' %%[-+]?[0-9]+([.][0-9]+)?([Ee][-+]?[0-9]+)?%%'identifier' %%[a-zA-Z_][a-zA-Z0-9_]*%%%grammarName GLSL
%blockComment on
%inlineComment on有了解析器，就有了单词流List<Token>和语法树Node。面对GLSL代码的语法树，我们如何对源代码进行格式化呢？
格式化的目的是让源代码从书写上更适宜人类阅读。具体来说，格式化要做的事，只是增减某些空白符（空格、tab符、换行符）而已，不会修改具有语法意义的内容。
在C#中，
if (x >0) {t = x;x = 0;
}会被格式化为
if (x > 0) {t = x; x = 0;
}而
if (x > 0) {t = x;
x= 0;
}会被格式化为
if (x > 0) {t = x;x = 0;
}同样意义的源代码，格式化结果却不同。这有些复杂。
我们将问题拆开，一步一步来。
首先，我们忽略多行注释blockComment、单行注释inlineComment和源代码中原有的空白符。
此时，一个if(x>0){t=x;x=0;}就应当被格式化为：
if (x > 0) {t = x;x = 0;
}也就是说，语法树中的每一部分，都可以自己判断出应当如何格式化。用递归的方式遍历语法树即可。
现在来考虑注释。注释可能出现在List<Token>（词法分析器的分析结果）里的任意位置上，属实调皮。于是就有这样一个想法，利用C#的yield return语法糖，我们执行下述算法：
遍历`List<Token>`：如果下一个要进行格式化的Token是注释，那么就在当前位置输出它并换行。如果下一个要进行格式化的Token不是注释，那么就用`yield return`的方式，在语法树上格式化地输出它（输出它+某些空白符）。只要在语法树每输出一个Token时，都进行一次yield return（return什么无所谓），那么yield return就能和遍历List<Token>的过程同步完成。
现在能够带着注释格式化了，但是无法解决这样的情况：上一行和下一行代码之间有好几个空行（即换行符），上述格式化过程会无视之。
完全的格式化算法要继承上述方法的思路，并加强如下：
语法树的各类语法块`Node`分别实现自己内部的格式化办法。
语法块`Node`要根据上一级`Node`传递来的要求，在输出自己内部的第一个`Token`之前，先输出多少个换行符和空格。
语法块`Node`要告知自己的每个下一级`Node`，它希望这些子级先输出多少个换行符和空格。
每个`Token`根据（上级传递来的要求+它与前一个`Token`之间的位置关系（间隔多少空格和换行符））输出自己前面的空白符，而后输出自己的内容。每个语法块`Node`都服从直接上级的要求，并对各个直接下级发出要求。通过这样接力的方式，就可以通过遍历一次语法树来输出格式化的代码了。
同学们可以下载链接中的工具查看效果，里面还有一些示例shader。
