Uyuw's Concert POJ2451

裸半平面交，以前没写过，先写一遍再说

我越来越不注意细节了，最后才发现空间稍微开小了（没有开那个零头，他又要多4条边，就WA了）

  1 const

  2     maxn=20010;

  3     eps=1e-7;

  4 type

  5     point=record

  6       x,y:double;

  7     end;

  8     poly=record

  9       x,y,w:point;

 10     end;

 11 

 12 var

 13     a:array[0..maxn]of poly;

 14     n:longint;

 15 

 16 operator -(a,b:point)c:point;

 17 begin

 18     c.x:=a.x-b.x;

 19     c.y:=a.y-b.y;

 20 end;

 21 

 22 operator +(a,b:point)c:point;

 23 begin

 24     c.x:=a.x+b.x;

 25     c.y:=a.y+b.y;

 26 end;

 27 

 28 operator *(a:point;b:double)c:point;

 29 begin

 30     c.x:=a.x*b;

 31     c.y:=a.y*b;

 32 end;

 33 

 34 operator *(a,b:point)c:double;

 35 begin

 36     c:=a.x*b.y-a.y*b.x;

 37 end;

 38 

 39 function wh(a:point):longint;

 40 begin

 41     with a do

 42       begin

 43         if abs(x)<eps then

 44           if y>eps then exit(2)

 45           else exit(4)

 46         else

 47           if abs(y)<eps then

 48             if x>eps then exit(1)

 49             else exit(3)

 50           else

 51             if x>eps then

 52               if y>eps then exit(1)

 53               else exit(4)

 54             else

 55               if y>eps then exit(2)

 56               else exit(3);

 57       end;

 58 end;

 59 

 60 procedure swap(var x,y:poly);

 61 var

 62     t:poly;

 63 begin

 64     t:=x;x:=y;y:=t;

 65 end;

 66 

 67 procedure sort(l,r:longint);

 68 var

 69     i,j:longint;

 70     y:point;

 71 begin

 72     i:=l;

 73     j:=r;

 74     y:=a[(l+r)>>1].w;

 75     repeat

 76       while (wh(a[i].w)<wh(y)) or ((wh(a[i].w)=wh(y)) and (y*a[i].w+eps<0)) do

 77         inc(i);

 78       while (wh(a[j].w)>wh(y)) or ((wh(a[j].w)=wh(y)) and (y*a[j].w>eps)) do

 79         dec(j);

 80       if i<=j then

 81       begin

 82         swap(a[i],a[j]);

 83         inc(i);

 84         dec(j);

 85       end;

 86     until i>j;

 87     if i<r then sort(i,r);

 88     if j>l then sort(l,j);

 89 end;

 90 

 91 procedure init;

 92 var

 93     i:longint;

 94 begin

 95     read(n);

 96     for i:=1 to n do

 97       with a[i] do

 98         begin

 99           read(x.x,x.y,y.x,y.y);

100           w:=y-x;

101         end;

102     a[n+2].x.x:=10000;

103     a[n+3].x.x:=10000;

104     a[n+3].x.y:=10000;

105     a[n+4].x.y:=10000;

106     for i:=0 to 3 do

107       begin

108         a[n+i+1].y:=a[n+((i+1)mod 4)+1].x;

109         a[n+i+1].w:=a[n+i+1].y-a[n+i+1].x;

110       end;

111     inc(n,4);

112     sort(1,n);

113 end;

114 

115 function get(a,b:poly):point;

116 var

117     s1,s2:double;

118 begin

119     s1:=a.w*(b.y-a.x);

120     s2:=a.w*(b.x-a.x);

121     if s1*s2>eps then s1:=-s1

122     else

123       begin

124         s1:=abs(s1);

125         s2:=abs(s2);

126       end;

127     if abs(s1+s2)<eps then get:=b.x

128     else get:=b.x+(b.w*(s2/(s1+s2)));

129 end;

130 

131 var

132     q:array[0..maxn]of poly;

133     d:array[0..maxn]of point;

134     l,r:longint;

135     ans:double;

136 

137 procedure work;

138 var

139     i:longint;

140 begin

141     l:=1;

142     r:=1;

143     q[1]:=a[1];

144     for i:=2 to n do

145       begin

146         while (l<r) and (a[i].w*(d[r-1]-a[i].x)+eps<0) do

147           dec(r);

148         while (l<r) and (a[i].w*(d[l]-a[i].x)+eps<0) do

149           inc(l);

150         inc(r);

151         q[r]:=a[i];

152         if abs(q[r-1].w*q[r].w)<eps then

153         begin

154           dec(r);

155           if a[i].w*(q[r].x-a[i].x)<0 then q[r]:=a[i];

156         end;

157         d[r-1]:=get(q[r],q[r-1]);

158       end;

159     while (l<r) and (q[l].w*(d[r-1]-q[l].x)+eps<0) do

160       dec(r);

161     if l<r then d[r]:=get(q[l],q[r]);

162     if r-l<=1 then writeln('0.0')

163     else

164       begin

165         for i:=l to r-1 do

166           ans:=ans+(d[i]*d[i+1])/2;

167         ans:=ans+(d[r]*d[l])/2;

168         ans:=abs(ans);

169         if ans<eps then writeln('0.0')

170         else writeln(ans:0:1);

171       end;

172 end;

173 

174 begin

175     init;

176     work;

177 end.

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